September 11 attacks

The September 11 attacks, commonly known as 9/11, were four coordinated Islamist terrorist suicide attacks carried out by al-Qaeda against the United States of America in 2001. That morning, 19 terrorists hijacked four commercial airliners scheduled to travel from the East Coast to California. The hijackers crashed the first two planes into the Twin Towers of the World Trade Center in New York City, two of the world's five tallest buildings at the time, and aimed the next two flights toward targets in or near Washington, D.C., in an attack on the nation's capital. The third team succeeded in striking the Pentagon, the headquarters of the U.S. Department of Defense in Arlington County, Virginia, while the fourth plane crashed in rural Pennsylvania during a passenger revolt. The September 11 attacks killed 2,977 people, making them the deadliest terrorist attack in history. In response to the attacks, the United States waged the multi-decade global War on Terror to eliminate hostile groups deemed terrorist organizations, as well as the foreign governments purported to support them. Conflicts were fought in Afghanistan, Iraq, Syria, and several other countries, under this justification.

The first impact was that of American Airlines Flight 11, which ringleader Mohamed Atta flew into the North Tower of the World Trade Center complex in Lower Manhattan at 8:46 a.m. Seventeen minutes later, at 9:03, the World Trade Center's South Tower was hit by United Airlines Flight 175. Both 110-story skyscrapers collapsed within an hour and forty-one minutes, bringing about the destruction of the remaining five structures in the WTC complex and damaging or destroying nearby buildings. A third flight, American Airlines Flight 77, flew towards Washington, D.C. and crashed into the Pentagon at 9:37 a.m., causing a partial collapse. The fourth and final flight, United Airlines Flight 93, also changed course towards Washington, believed by investigators to target either the United States Capitol or the White House. Alerted to the previous attacks, the passengers revolted against the hijackers; the hijackers crashed the aircraft into a Stonycreek Township field, near Shanksville, Pennsylvania, at 10:03 a.m. The Federal Aviation Administration ordered an indefinite ground stop for all air traffic in U.S. airspace at 9:45 a.m. (59 minutes following the first impact), preventing any further aircraft departures until September 13 and requiring all airborne aircraft to return to their point of origin or divert to Canada; the actions undertaken in Canada to support incoming aircraft and their occupants were collectively titled Operation Yellow Ribbon.

That evening, the Central Intelligence Agency informed President George W. Bush that its Counterterrorism Center had identified the attacks as having been the work of Al-Qaeda under Osama bin Laden's leadership. The United States formally responded by launching the war on terror and invading Afghanistan to depose the Taliban, which rejected the conditions of U.S. terms to expel Al-Qaeda from Afghanistan and extradite its leaders. The U.S.'s invocation of Article 5 of the North Atlantic Treaty—its only usage to date—called upon allies to fight Al-Qaeda. As U.S. and NATO invasion forces swept through Afghanistan, bin Laden eluded them by disappearing into the White Mountains. He denied any involvement until 2004, when excerpts of a taped statement in which he accepted responsibility for the attacks were released. Al-Qaeda's cited motivations included U.S. support of Israel, the presence of U.S. military bases in Saudi Arabia and sanctions against Iraq. The nearly decade-long manhunt for bin Laden concluded on May 2, 2011, when he was killed during a U.S. military raid after being tracked down to his compound in Abbottabad, Pakistan. The war in Afghanistan continued for another eight years until the agreement was made in February 2020 for American and NATO troops to withdraw from the country. The last members of the U.S. armed forces left the region on August 30, 2021, after which the Taliban rapidly returned to power. Ayman al-Zawahiri, another planner of the attacks who succeeded bin Laden as leader of Al-Qaeda, was killed by U.S. drone strikes in Kabul, Afghanistan on July 31, 2022.

Excluding the hijackers, the attacks killed 2,977 people, injured thousands more and gave rise to substantial long-term health consequences while also causing at least $10   billion in infrastructure and property damage. It remains the deadliest terrorist attack in history as well as the deadliest incident for firefighters and law enforcement personnel in American history, killing 343 and 72 members, respectively. The loss of life stemming from the impact of Flight 11 secured its place as the most lethal plane crash in aviation history followed by the death toll incurred by Flight 175. The destruction of the World Trade Center and its environs seriously harmed the U.S. economy and induced global market shocks. Many other countries strengthened anti-terrorism legislation and expanded their powers of law enforcement and intelligence agencies. The total number of deaths caused by the attacks, combined with the death tolls from the conflicts they directly incited, has been estimated by the Costs of War Project to be over 4.5 million people. Cleanup of the World Trade Center site (colloquially "Ground Zero") took eight months and was completed in May 2002, while the Pentagon was repaired within a year. After delays in the design of a replacement complex, six new buildings were planned to replace the lost towers, along with a museum and memorial dedicated to those who were killed or injured in the attacks. The tallest building, One World Trade Center, began construction in November 2006; it opened in November 2014. Memorials to the attacks include the National September 11 Memorial & Museum in New York City, the Pentagon Memorial in Arlington County, Virginia, and the Flight 93 National Memorial at the Pennsylvania crash site.

Al-Qaeda
Al-Qaeda's origins can be traced to 1979, when the Soviet Union invaded Afghanistan. Soon after the invasion, Osama bin Laden traveled to Afghanistan and helped organize Arab mujahideen (the "Afghan Arabs") to resist the "Communist invaders" (Soviets) until their exit from the country in 1989. In 1984, bin Laden, along with Islamic scholar Abdullah Azzam, formed the Maktab al-Khidamat (MAK), an organization to support Arab mujahideen who came to join the jihad in Afghanistan.

The U.S. Central Intelligence Agency (CIA) funnelled several billion dollars worth of weapons to the indigenous Afghan mujahideen resistance, a portion of which bled to the Arab volunteers. However, no direct evidence of U.S. aid to bin Laden or any of his affiliates was ever uncovered.

In 1996, bin Laden issued his first fatwā, which declared war against the United States and demanded the expulsion of all American soldiers from the Arabian Peninsula. In a second 1998 fatwā, bin Laden outlined his objections to American foreign policy with respect to the State of Israel, as well as the continued presence of American troops in Saudi Arabia after the Gulf War. Bin Laden maintained that Muslims are obliged to attack American targets until the aggressive policies of the U.S. against Muslims were reversed. According to bin Laden, Islamic jurists had "throughout Islamic history unanimously agreed that the jihad is an individual duty if the enemy destroys the Muslim countries".

The Hamburg cell in Germany included Islamists who eventually came to be key operatives in the 9/11 attacks. Mohamed Atta; Marwan al-Shehhi; Ziad Jarrah; Ramzi bin al-Shibh; and Said Bahaji were all members of Al-Qaeda's Hamburg cell. Bin Laden asserted that all Muslims must wage a defensive war against the United States and combat American aggression. He further argued that military strikes against American assets would send a message to the American people, attempting to force the U.S. to re-evaluate its support to Israel, and other aggressive policies. In a 1998 interview with American journalist John Miller, bin Laden stated:

"We do not differentiate between those dressed in military uniforms and civilians; they are all targets in this fatwa. American history does not distinguish between civilians and military, not even women and children. They are the ones who used bombs against Nagasaki. Can these bombs distinguish between infants and military? America does not have a religion that will prevent it from destroying all people. So we tell the Americans as people and we tell the mothers of soldiers and American mothers in general that if they value their lives and the lives of their children, to find a nationalistic government that will look after their interests and not the interests of the Jews. The continuation of tyranny will bring the fight to America, as [the 1993 World Trade Center bomber] Ramzi [Yousef] yourself and others did. This is my message to the American people: to look for a serious government that looks out for their interests and does not attack others, their lands, or their honour. My word to American journalists is not to ask why we did that but to ask what their government has done that forced us to defend ourselves."

Osama bin Laden


Bin Laden orchestrated the September 11 attacks. He initially denied involvement, but later recanted his denial. Al Jazeera broadcast a statement by him on September 16, 2001: "I stress that I have not carried out this act, which appears to have been carried out by individuals with their own motivation". In November 2001, U.S. forces recovered a videotape from a destroyed house in Jalalabad, Afghanistan. In the video, bin Laden, talking to Khaled al-Harbi, admitted foreknowledge of the attacks. On December 27, 2001, a second video of bin Laden was released in which he, stopping short of admitting responsibility for the attacks, said: "It has become clear that the West in general and America in particular have an unspeakable hatred for Islam.  ... It is the hatred of crusaders. Terrorism against America deserves to be praised because it was a response to injustice, aimed at forcing America to stop its support for Israel, which kills our people.    ... We say that the end of the United States is imminent, whether Bin Laden or his followers are alive or dead, for the awakening of the Muslim ummah [nation] has occurred.    ... It is important to hit the economy (of the United States), which is the base of its military power...If the economy is hit they will become reoccupied."

Shortly before the 2004 U.S. presidential election, bin Laden used a taped statement to publicly acknowledge Al-Qaeda's involvement in the attacks. He admitted his direct link to the attacks and said they were carried out because:

"The events that affected my soul in a direct way started in 1982 when America permitted the Israelis to invade Lebanon and the American Sixth Fleet helped them in that. This bombardment began and many were killed and injured and others were terrorised and displaced.

I couldn't forget those moving scenes, blood and severed limbs, women and children sprawled everywhere. Houses were destroyed along with their occupants, high rises demolished over their residents, rockets raining down on our home without mercy...As I looked at those demolished towers in Lebanon, it entered my mind that we should punish the oppressor in kind and that we should destroy towers in America so that they taste some of what we tasted and so that they be deterred from killing our women and children.

And that day, it was confirmed to me that oppression and the intentional killing of innocent women and children is a deliberate American policy. Destruction is freedom and democracy, while resistance is terrorism and intolerance." Bin Laden personally directed his followers to attack the World Trade Center and the Pentagon. Another video obtained by Al Jazeera in September 2006 showed bin Laden with one of the attacks' chief planners, Ramzi bin al-Shibh, as well as hijackers, Hamza al-Ghamdi and Wail al-Shehri, amidst making preparations for the attacks. Bin Laden had been on the Federal Bureau of Investigation (FBI) Most Wanted List since 1998 for the 1998 bombings of the U.S. embassies in Tanzania and Kenya.

Khalid Sheikh Mohammed and other Al-Qaeda members
Journalist Yosri Fouda of the Arabic television channel Al Jazeera reported that in April 2002 Al-Qaeda member Khalid Sheikh Mohammed admitted his involvement in the attacks, along with Ramzi bin al-Shibh. The 2004 9/11 Commission Report determined that the animosity which Mohammed, the principal architect of the 9/11 attacks, felt towards the United States had stemmed from his "violent disagreement with U.S. foreign policy favoring Israel". Mohammed was also an adviser and financier of the 1993 World Trade Center bombing and the uncle of Ramzi Yousef, the lead bomber in that attack. In late 1994, Mohammed and Yousef moved on to plan a new terrorist attack called the Bojinka plot planned for January 1995. Despite a failure and Yousef's capture by U.S. forces the following month, the Bojinka plot would influence the later 9/11 attacks.

In "Substitution for Testimony of Khalid Sheikh Mohammed" from the trial of Zacarias Moussaoui, five people are identified as having been completely aware of the operation's details. They are bin Laden; Khalid Sheikh Mohammed; Ramzi bin al-Shibh; Abu Turab al-Urduni; and Mohammed Atef.

Motives
Osama bin Laden's declaration of a holy war against the United States, and a 1998 fatwā signed by bin Laden and others that called for the killing of Americans, are seen by investigators as evidence of his motivation. During his interview with Hamid Mir in November 2001, Bin Laden defended the September 11 attacks as retaliatory strikes against American atrocities against Muslims across the world. He also maintained that the attacks were not directed against women and children, asserting that the targets of the strikes were symbols of America's "economic and military power".

In bin Laden's November 2002 "Letter to the American people", he explicitly stated that al-Qaeda's motives for the attacks included:
 * U.S. support of Israel
 * Bin Laden's strategy to support and globally expand the Al-Aqsa Intifada
 * Attacks against Muslims by U.S.-led coalition in Somalia
 * U.S. support of the government of Philippines against Muslims in the Moro conflict
 * U.S. support for the Israeli occupation of Southern Lebanon
 * U.S. support of Russian atrocities against Muslims in Chechnya
 * Pro-American governments in the Middle East (who "act as your agents") being against Muslim interests
 * U.S. support of Indian oppression against Muslims in Kashmir
 * The presence of U.S. troops in Saudi Arabia *Plotz, David (2001) What Does Osama Bin Laden Want?, Slate
 * The sanctions against Iraq
 * Environmental destruction
 * The sanctions against Iraq
 * Environmental destruction
 * The sanctions against Iraq
 * Environmental destruction
 * The sanctions against Iraq
 * Environmental destruction

After the attacks, bin Laden and Ayman al-Zawahiri released additional videotapes and audio recordings, some of which repeated the above reasons for the attacks. Two relevant publications were bin Laden's 2002 "Letter to the American people" and a 2004 videotape by bin Laden.

As an adherent of Islam, bin Laden believed that non-Muslims are forbidden from having a permanent presence in the Arabian Peninsula. In 1996, bin Laden issued a fatwā calling for American troops to leave Saudi Arabia. In 1998, Al-Qaeda wrote "For over seven years the United States has been occupying the lands of Islam in the holiest of places, the Arabian Peninsula, plundering its riches, dictating to its rulers, humiliating its people, terrorizing its neighbours, and turning its bases in the Peninsula into a spearhead through which to fight the neighbouring Muslim peoples".

In a December 1999 interview, bin Laden said he felt that Americans were "too near to Mecca", and considered this a provocation to the entire Muslim world. One analysis of suicide terrorism suggested that without U.S. troops in Saudi Arabia, Al-Qaeda likely would not have been able to get people to commit to suicide missions.

In the 1998 fatwā, Al-Qaeda identified the Iraq sanctions as a reason to kill Americans, condemning the "protracted blockade" among other actions that constitute a declaration of war against "Allah, his messenger, and Muslims". The fatwā declared that "the ruling to kill the Americans and their allies—civilians and military—is an individual duty for every Muslim who can do it in any country in which it is possible to do it, in order to liberate the al-Aqsa Mosque and the holy mosque of Mecca from their grip, and in order for their [the Americans'] armies to move out of all the lands of Islam, defeated and unable to threaten any Muslim".

In 2004, bin Laden claimed that the idea of destroying the towers had first occurred to him in 1982 when he witnessed Israel's bombardment of high-rise apartment buildings during the 1982 Lebanon War. Some analysts, including political scientists John Mearsheimer and Stephen Walt, also claimed that U.S. support of Israel was a motive for the attacks. In 2004 and 2010, bin Laden again connected the September 11 attacks with U.S. support of Israel, although most of the letters expressed bin Laden's disdain for President Bush and bin Laden's hope to "destroy and bankrupt" the U.S.

Other motives have been suggested in addition to those stated by bin Laden and Al-Qaeda. Some authors suggested the "humiliation" that resulted from the Islamic world falling behind the Western world—this discrepancy was rendered especially visible by globalization and a desire to provoke the U.S. into a broader war against the Islamic world in the hope of motivating more allies to support Al-Qaeda. Similarly, others have argued the 9/11 attacks were a strategic move to provoke America into a war that would incite a pan-Islamic revolution.

Documents seized during the 2011 operation that killed bin Laden included a few notes handwritten by bin Laden in September 2002 with the heading "The Birth of the Idea of September 11". In these notes, he describes how he was inspired by the crash of EgyptAir Flight 990 on October 31, 1999, which was deliberately crashed by co-pilot Gameel Al-Batouti. "This is how the idea of 9/11 was conceived and developed in my head, and that is when we began the planning" bin Laden continued, adding that no one but Abu Hafs and Abu al-Khair knew about it at the time. The 9/11 Commission Report identified Khalid Sheikh Mohammed as the architect of 9/11, but he is not mentioned in bin Laden's notes.

Planning
The attacks were conceived by Khalid Sheikh Mohammed, who first presented it to Osama bin Laden in 1996. At that time, bin Laden and Al-Qaeda were in a period of transition, having just relocated back to Afghanistan from Sudan. The 1998 African embassy bombings and bin Laden's February 1998 fatwā marked a turning point of Al-Qaeda's terrorist operation, as bin Laden became intent on attacking the United States.

In late 1998 or early 1999, bin Laden approved Mohammed to go forward with organizing the plot. Mohammed, bin Laden, and Mohammed Atef, bin Laden's deputy, held a series of meetings in early 1999. Atef provided operational support, including target selections and helping arrange travel for the hijackers. Bin Laden overruled Mohammed, rejecting potential targets such as the U.S. Bank Tower in Los Angeles for lack of time.

Bin Laden provided leadership and financial support and was involved in selecting participants. He initially selected Nawaf al-Hazmi and Khalid al-Mihdhar, both experienced jihadists who had fought in Bosnia. Hazmi and Mihdhar arrived in the United States in mid-January 2000. In early 2000, Hazmi and Mihdhar took flying lessons in San Diego, California. Both spoke little English, performed poorly in flying lessons, and eventually served as secondary "muscle" hijackers.

In late 1999, a group of men from Hamburg, Germany, arrived in Afghanistan. The group included Mohamed Atta, Marwan al-Shehhi, Ziad Jarrah, and Ramzi bin al-Shibh. Bin Laden selected these men because they were educated, could speak English, and had experience living in the West. New recruits were routinely screened for special skills and Al-Qaeda leaders consequently discovered that Hani Hanjour already had a commercial pilot's license. Mohammed later said that he helped the hijackers blend in by teaching them how to order food in restaurants and dress in Western clothing.

Hanjour arrived in San Diego on December 8, 2000, joining Hazmi. They soon left for Arizona, where Hanjour took refresher training. Marwan al-Shehhi arrived at the end of May 2000, while Atta arrived on June 3, 2000, and Jarrah arrived on June 27, 2000. Bin al-Shibh applied several times for a visa to the United States, but as a Yemeni, he was rejected out of concerns he would overstay his visa. Bin al-Shibh stayed in Hamburg, providing coordination between Atta and Mohammed. The three Hamburg cell members all took pilot training in South Florida at Huffman Aviation.

In the spring of 2001, the secondary hijackers began arriving in the United States. In July 2001, Atta met with bin al-Shibh in Tarragona, Catalonia, Spain, where they coordinated details of the plot, including final target selection. Bin al-Shibh also passed along bin Laden's wish for the attacks to be carried out as soon as possible. Some of the hijackers received passports from corrupt Saudi officials who were family members or used fraudulent passports to gain entry.

There have been a few theories that 9/11 was selected by the hijackers as the date of the attack because it resembled 9-1-1, the phone number used to report emergencies in the United States. However, Lawrence Wright wrote that the hijackers chose the date when John III Sobieski, the King of Poland and Grand Duke of Lithuania, began the battle that turned back the Ottoman Empire's Muslim armies that were attempting to capture Vienna (present-day capital of Austria) on September 11, 1683. During 1683, Vienna was the seat of the Holy Roman Empire and Habsburg monarchy, both major powers in Europe at the time. For Osama bin Laden, this was a date when the West gained some dominance over Islam, and by attacking on this date, he hoped to make a step in Islam "winning" the war for worldwide power and influence.

Prior intelligence
In late 1999, Al-Qaeda associate Walid bin Attash ("Khallad") contacted Mihdhar and told him to meet in Kuala Lumpur, Malaysia; Hazmi and Abu Bara al Yemeni would also be in attendance. The NSA intercepted a telephone call mentioning the meeting, Mihdhar, and the name "Nawaf" (Hazmi); while the agency feared "Something nefarious might be afoot", it took no further action.

The CIA had already been alerted by Saudi intelligence about the status of Mihdhar and Hazmi as Al-Qaeda members and a CIA team broke into Mihdhar's Dubai hotel room and discovered that Mihdhar had a U.S. visa. While Alec Station alerted intelligence agencies worldwide about this fact, it did not share this information with the FBI. The Malaysian Special Branch observed the January 5, 2000, meeting of the two Al-Qaeda members and informed the CIA that Mihdhar, Hazmi, and Khallad were flying to Bangkok, but the CIA never notified other agencies of this, nor did it ask the State Department to put Mihdhar on its watchlist. An FBI liaison to Alec Station asked permission to inform the FBI of the meeting but was told: "This is not a matter for the FBI".

By late June, senior counter-terrorism official Richard Clarke and CIA director George Tenet were "convinced that a major series of attacks was about to come", although the CIA believed the attacks would likely occur in Saudi Arabia or Israel. In early July, Clarke put domestic agencies on "full alert", telling them, "Something spectacular is going to happen here, and it's going to happen soon". He asked the FBI and the State Department to alert the embassies and police departments, and the Defense Department to go to "Threat Condition Delta". Clarke later wrote: "Somewhere in CIA there was information that two known al Qaeda terrorists had come into the United States. Somewhere in the FBI, there was information that strange things had been going on at flight schools in the United States.   [...] They had specific information about individual terrorists from which one could have deduced what was about to happen. None of that information got to me or the White House".

On July 13, Tom Wilshire, a CIA agent assigned to the FBI's international terrorism division, emailed his superiors at the CIA's Counterterrorism Center (CTC) requesting permission to inform the FBI that Hazmi was in the country and that Mihdhar had a U.S. visa. The CIA never responded.

The same day in July, Margarette Gillespie, an FBI analyst working in the CTC, was told to review material about the Malaysia meeting. She was not told of the participant's presence in the U.S. The CIA gave Gillespie surveillance photos of Mihdhar and Hazmi from the meeting to show to FBI counterterrorism but did not tell her their significance. The Intelink database informed her not to share intelligence material at the meeting with criminal investigators. When shown the photos, the FBI refused more details on their significance, and they were not given Mihdhar's date of birth or passport number. In late August 2001, Gillespie told the INS, the State Department, the Customs Service, and the FBI to put Hazmi and Mihdhar on their watchlists, but the FBI was prohibited from using criminal agents in searching for the duo, hindering their efforts.

Also in July, a Phoenix-based FBI agent sent a message to FBI headquarters, Alec Station, and FBI agents in New York alerting them to "the possibility of a coordinated effort by Osama bin Laden to send students to the United States to attend civil aviation universities and colleges". The agent, Kenneth Williams, suggested the need to interview all flight school managers and identify all Arab students seeking flight training. In July, Jordan alerted the U.S. that Al-Qaeda was planning an attack on the U.S.; "months later", Jordan notified the U.S. that the attack's codename was "The Big Wedding" and that it involved aeroplanes.

On August 6, 2001, the CIA's Presidential Daily Brief ("PDB"), designated "For the President Only", was entitled Bin Ladin Determined to Strike in US. The memo noted that FBI information "indicates patterns of suspicious activity in this country consistent with preparations for hijackings or other types of attacks".

In mid-August, one Minnesota flight school alerted the FBI about Zacarias Moussaoui, who had asked "suspicious questions". The FBI found that Moussaoui was a radical who had travelled to Pakistan, and the INS arrested him for overstaying his French visa. Their request to search his laptop was denied by FBI headquarters due to the lack of probable cause.

The failures in intelligence-sharing were attributed to 1995 Justice Department policies limiting intelligence-sharing, combined with CIA and NSA reluctance to reveal "sensitive sources and methods" such as tapped phones. Testifying before the 9/11 Commission in April 2004, then—Attorney General John Ashcroft recalled that the "single greatest structural cause for the September 11th problem was the wall that segregated or separated criminal investigators and intelligence agents". Clarke also wrote: "[T]here were... failures to get information to the right place at the right time".

Attacks
Early on the morning of Tuesday, September 11, 2001, nineteen hijackers took control of four commercial airliners (two Boeing 757s and two Boeing 767s) en route to California after takeoffs from Logan International Airport in Boston, Massachusetts; Newark Liberty International Airport in Newark, New Jersey; and Washington Dulles International Airport in Loudoun and Fairfax counties in Virginia. Large planes with long flights were selected for hijacking because they would have more fuel. Atta said to hijack Boeing aircraft based on his understanding of the autopilot feature. * Eastern Daylight Time (UTC−04:00) † Excluding hijackers § Including emergency workers ‡ Including hijackers

The four crashes
At 7:59 a.m., American Airlines Flight 11 took off from Logan International Airport in Boston. Fifteen minutes into the flight, five hijackers armed with boxcutters took over the plane, injuring at least three people (and possibly killing one)  before forcing their way into the cockpit. The terrorists also displayed an apparent explosive and sprayed mace into the cabin, to frighten the hostages into submission and further hinder resistance. Back at Logan, United Airlines Flight 175 took off at 8:14 a.m., approximately the same time as Flight 11's hijacking. Hundreds of miles southwest at Dulles International Airport, American Airlines Flight 77 left the runway at 8:20 a.m. Flight 175's journey proceeded normally for 28 minutes until 8:42 a.m., when a group of five hijacked the plane, murdering both pilots and stabbing several crew members before assuming control of the aircraft. These hijackers also used bomb threats to instil fear into the passengers and crew, also spraying chemical weapons to disable any opposition. Concurrently, United Airlines Flight 93 departed from Newark International Airport in New Jersey; originally scheduled to pull away from the gate at 8:00 a.m., the plane was running 42 minutes late.

At 8:46 a.m., Flight 11 was deliberately crashed into the north face of the World Trade Center's North Tower (1 WTC), although the initial presumption by many was that it was an accident. At 8:51 a.m., American Airlines Flight 77 was also taken over by another group of five who forcibly entered the cockpit 31 minutes after takeoff. Although the hijackers on this flight were equipped with knives, there were no reports of anyone on board being stabbed, nor did the two people who made phone calls mention the use of mace or a bomb threat.

Seventeen minutes after the first plane crashed into the North Tower, Flight 175 was flown into the South Tower's southern facade (2   WTC) at 9:03 a.m., demonstrating that the first crash was not an accident, but rather a terrorist attack.

Four men aboard Flight 93 struck suddenly, killing at least one passenger, after having waited 46 minutes to make their move—a holdup that proved disastrous for the terrorists when combined with the delayed takeoff from the runway; they stormed the cockpit and seized control of the plane at 9:28 a.m., turning the plane eastbound and setting course for Washington, D.C. Much like their counterparts on the first two flights, the fourth team also used bomb threats and filled the cabin with mace.

Nine minutes after Flight 93's hijacking, Flight 77 crashed into the west side of the Pentagon. Because of the two delays, the passengers and crew of Flight 93 had time to be made aware of the previous attacks through phone calls to the ground, and as a result an uprising was hastily organized to take control of the aircraft at 9:57 a.m. Within minutes, passengers had fought their way to the front of the cabin and began breaking down the cockpit door. Fearing their captives would gain the upper hand, the hijackers rolled the plane and pitched it into a nosedive, crashing into a field near Shanksville, Pennsylvania, southeast of Pittsburgh, at 10:03 a.m. The plane was about twenty minutes away from reaching D.C. at the time of the crash, and its target is believed to have been either the Capitol Building or the White House.

Some passengers and crew members who called from the aircraft using the cabin air phone service and mobile phones provided details: several hijackers were aboard each plane; they used mace, tear gas, or pepper spray to overcome attendants; and some people aboard had been stabbed. *
 * Summers and Swan (2011), pp. 58, 463n, 476n.
 * Reports indicated hijackers stabbed and killed pilots, flight attendants, and one or more passengers. According to the 9/11 Commission's final report, the hijackers had recently purchased multi-function hand tools and assorted Leatherman-type utility knives with locking blades (which were not forbidden to passengers at the time), but these were not found among the possessions left behind by the hijackers. A flight attendant on Flight 11, a passenger on Flight 175, and passengers on Flight 93 said the hijackers had bombs, but one of the passengers said he thought the bombs were fake. The FBI found no traces of explosives at the crash sites, and the 9/11 Commission concluded that the bombs were probably fake. On at least two of the hijacked flights—American 11 and United 93—the terrorists claimed over the PA system that they were taking hostages and were returning to the airport to have a ransom demand met, a clear attempt to prevent passengers from fighting back. Both attempts failed, however, as both hijacker pilots in these instances (Mohamed Atta and Ziad Jarrah, respectively) keyed the wrong switch and mistakenly transmitted their messages to ATC instead of the people on the plane as intended, tipping off the flight controllers that the planes had been hijacked.
 * Reports indicated hijackers stabbed and killed pilots, flight attendants, and one or more passengers. According to the 9/11 Commission's final report, the hijackers had recently purchased multi-function hand tools and assorted Leatherman-type utility knives with locking blades (which were not forbidden to passengers at the time), but these were not found among the possessions left behind by the hijackers. A flight attendant on Flight 11, a passenger on Flight 175, and passengers on Flight 93 said the hijackers had bombs, but one of the passengers said he thought the bombs were fake. The FBI found no traces of explosives at the crash sites, and the 9/11 Commission concluded that the bombs were probably fake. On at least two of the hijacked flights—American 11 and United 93—the terrorists claimed over the PA system that they were taking hostages and were returning to the airport to have a ransom demand met, a clear attempt to prevent passengers from fighting back. Both attempts failed, however, as both hijacker pilots in these instances (Mohamed Atta and Ziad Jarrah, respectively) keyed the wrong switch and mistakenly transmitted their messages to ATC instead of the people on the plane as intended, tipping off the flight controllers that the planes had been hijacked.
 * Reports indicated hijackers stabbed and killed pilots, flight attendants, and one or more passengers. According to the 9/11 Commission's final report, the hijackers had recently purchased multi-function hand tools and assorted Leatherman-type utility knives with locking blades (which were not forbidden to passengers at the time), but these were not found among the possessions left behind by the hijackers. A flight attendant on Flight 11, a passenger on Flight 175, and passengers on Flight 93 said the hijackers had bombs, but one of the passengers said he thought the bombs were fake. The FBI found no traces of explosives at the crash sites, and the 9/11 Commission concluded that the bombs were probably fake. On at least two of the hijacked flights—American 11 and United 93—the terrorists claimed over the PA system that they were taking hostages and were returning to the airport to have a ransom demand met, a clear attempt to prevent passengers from fighting back. Both attempts failed, however, as both hijacker pilots in these instances (Mohamed Atta and Ziad Jarrah, respectively) keyed the wrong switch and mistakenly transmitted their messages to ATC instead of the people on the plane as intended, tipping off the flight controllers that the planes had been hijacked.
 * Reports indicated hijackers stabbed and killed pilots, flight attendants, and one or more passengers. According to the 9/11 Commission's final report, the hijackers had recently purchased multi-function hand tools and assorted Leatherman-type utility knives with locking blades (which were not forbidden to passengers at the time), but these were not found among the possessions left behind by the hijackers. A flight attendant on Flight 11, a passenger on Flight 175, and passengers on Flight 93 said the hijackers had bombs, but one of the passengers said he thought the bombs were fake. The FBI found no traces of explosives at the crash sites, and the 9/11 Commission concluded that the bombs were probably fake. On at least two of the hijacked flights—American 11 and United 93—the terrorists claimed over the PA system that they were taking hostages and were returning to the airport to have a ransom demand met, a clear attempt to prevent passengers from fighting back. Both attempts failed, however, as both hijacker pilots in these instances (Mohamed Atta and Ziad Jarrah, respectively) keyed the wrong switch and mistakenly transmitted their messages to ATC instead of the people on the plane as intended, tipping off the flight controllers that the planes had been hijacked.
 * Reports indicated hijackers stabbed and killed pilots, flight attendants, and one or more passengers. According to the 9/11 Commission's final report, the hijackers had recently purchased multi-function hand tools and assorted Leatherman-type utility knives with locking blades (which were not forbidden to passengers at the time), but these were not found among the possessions left behind by the hijackers. A flight attendant on Flight 11, a passenger on Flight 175, and passengers on Flight 93 said the hijackers had bombs, but one of the passengers said he thought the bombs were fake. The FBI found no traces of explosives at the crash sites, and the 9/11 Commission concluded that the bombs were probably fake. On at least two of the hijacked flights—American 11 and United 93—the terrorists claimed over the PA system that they were taking hostages and were returning to the airport to have a ransom demand met, a clear attempt to prevent passengers from fighting back. Both attempts failed, however, as both hijacker pilots in these instances (Mohamed Atta and Ziad Jarrah, respectively) keyed the wrong switch and mistakenly transmitted their messages to ATC instead of the people on the plane as intended, tipping off the flight controllers that the planes had been hijacked.

Three buildings in the World Trade Center collapsed due to fire-induced structural failure. Although the South Tower was struck 17 minutes after the North Tower, the plane's impact zone was far lower, at a much faster speed, and into a corner, with the unevenly-balanced additional structural weight causing it to collapse first at 9:59 a.m.,  having burned for 56 minutes in the fire caused by the crash of United Airlines Flight 175 and the explosion of its fuel. The North Tower lasted another 29 minutes before collapsing at 10:28 a.m., one hour and forty-two minutes after being struck by American Airlines Flight 11. When the North Tower collapsed, debris fell on the nearby 7 World Trade Center building (7   WTC), damaging the building and starting fires. These fires burned for nearly seven hours, compromising the building's structural integrity, and 7   WTC collapsed at 5:21    p.m.  The west side of the Pentagon sustained significant damage.

At 9:42 a.m., the Federal Aviation Administration (FAA) grounded all civilian aircraft within the continental U.S., and civilian aircraft already in flight were told to land immediately. All international civilian aircraft were either turned back or redirected to airports in Canada or Mexico, and were banned from landing on United States territory for three days. The attacks created widespread confusion among news organizations and air traffic controllers. Among unconfirmed and often contradictory news reports aired throughout the day, one of the most prevalent claimed a car bomb had been detonated at the U.S. State Department's headquarters in Washington, D.C. Another jet (Delta Air Lines Flight 1989) was suspected of having been hijacked, but the aircraft responded to controllers and landed safely in Cleveland, Ohio.

In an April 2002 interview, Khalid Sheikh Mohammed and Ramzi bin al-Shibh, who are believed to have organized the attacks, said Flight 93's intended target was the United States Capitol, not the White House. During the planning stage of the attacks, Mohamed Atta (Flight 11's hijacker and pilot) thought the White House might be too tough a target and sought an assessment from Hani Hanjour (who hijacked and piloted Flight 77). Mohammed said Al-Qaeda initially planned to target nuclear installations rather than the World Trade Center and the Pentagon, but decided against it, fearing things could "get out of control". Final decisions on targets, according to Mohammed, were left in the hands of the pilots. If any pilot could not reach his intended target, he was to crash the plane.

Casualties
The attack on the World Trade Center's North Tower single-handedly made 9/11 the deadliest act of terrorism in world history. Taken together, the four crashes caused the deaths of 2,996 people (including the hijackers) and injured thousands more. The death toll included 265 on the four planes (from which there were no survivors); 2,606 in the World Trade Center and in the surrounding area; and 125 at The Pentagon. Most who died were civilians, as well as 343 firefighters, 72 law enforcement officers, 55 military personnel, and the 19 terrorists. After New York, New Jersey lost the most state citizens. More than 90 countries lost citizens in the attacks.

In New York City, more than 90% of the workers and visitors who died in the towers had been at or above the points of impact. In the North Tower, between 1,344 and 1,402 people were at, above or one floor below the point of impact and all died. Hundreds were killed instantly the moment the plane struck. The estimated 800 people who survived the impact were trapped and died in the fires or from smoke inhalation; fell or jumped from the tower to escape the smoke and flames; or were killed in the building's collapse. The destruction of all three staircases in the North Tower when Flight 11 hit made it impossible for anyone from the impact zone upward to escape. 107 people not trapped by the impact died. When the plane struck between floors 93 and 99, the 92nd floor was also rendered inescapable when the crash severed all elevator shafts while debris falling from the impact zone blocked the stairwells, ensuring the deaths of all 69 workers on the floor below the point of impact.

In the South Tower, around 600 people were on or above the 77th floor when Flight 175 struck and few survived. As with the North Tower, hundreds were killed at the moment of impact. Unlike those in the North Tower, the estimated 300 survivors of the crash were not technically trapped by the damage done by Flight 175's impact, but most were either unaware that a means of escape still existed or were unable to use it. One stairway, Stairwell A, narrowly avoided being destroyed as Flight 175 crashed through the building, allowing 14 people located on the floors of impact (including Stanley Praimnath, a man who saw the plane coming at him) and four more from the floors above to escape. New York City 9-1-1 operators who received calls from people inside the tower were not well informed of the situation as it rapidly unfolded and as a result, told callers not to descend the tower on their own. In total, 630 people died in the South Tower, fewer than half the number killed in the North Tower. Of the 100–200 people witnessed jumping or falling to their deaths that morning, only three recorded sightings were from the South Tower. Casualties in the South Tower were significantly reduced because some occupants decided to leave the building immediately following the first crash, and because Eric Eisenberg, an executive at AON Insurance, made the decision to evacuate the floors occupied by AON (floors 92 and 98–105) in the moments following the impact of Flight 11. The 17-minute gap allowed over 900 of the 1,100 AON employees present on-site to evacuate from above the 77th floor before the South Tower was struck; Eisenberg was among the nearly 200 who did not escape. Similar pre-impact evacuations were carried out by companies such as Fiduciary Trust, CSC, and Euro Brokers, all of whom had offices on floors above the point of impact. The failure to order a full evacuation of the South Tower after the first plane crash into the North Tower was described by USA Today as "one of the day's great tragedies".

As exemplified in the photograph The Falling Man, more than 200 people fell to their deaths from the burning towers, most of whom were forced to jump to escape the extreme heat, fire and smoke. Some occupants of each tower above the point of impact made their way toward the roof in the hope of helicopter rescue, but the roof access doors were locked. No plan existed for helicopter rescues, and the combination of roof equipment, thick smoke and intense heat prevented helicopters from approaching.

At the World Trade Center complex, a total of 414 emergency workers died as they tried to rescue people and fight fires, while another law enforcement officer was separately killed when United 93 crashed. The New York City Fire Department (FDNY) lost 343 firefighters, including a chaplain and two paramedics. The New York City Police Department (NYPD) lost 23 officers. The Port Authority Police Department (PAPD) lost 37 officers. Eight emergency medical technicians (EMTs) and paramedics from private emergency medical services (EMS) units were killed. Almost all of the emergency personnel who died at the scene that day were killed as a result of the towers collapsing, with the exception of one who was struck by a civilian falling from the upper floors of the South Tower.

Cantor Fitzgerald L.P. (an investment bank on the North Tower's 101st–105th floors) lost 658 employees, considerably more than any other employer. Marsh Inc., located immediately below Cantor Fitzgerald on floors 93–100, lost 358 employees, and 175 employees of Aon Corporation were also killed. The National Institute of Standards and Technology (NIST) estimated that about 17,400 civilians were in the World Trade Center complex at the time of the attacks. Turnstile counts from the Port Authority suggest 14,154 people were typically in the Twin Towers by 8:45   a.m. Most people below the impact zone safely evacuated the buildings.

In Arlington County, Virginia, 125 Pentagon workers died when Flight 77 crashed into the building's western side. 70 were civilians and 55 were military personnel, many of whom worked for the United States Army or the United States Navy. The Army lost 47 civilian employees; six civilian contractors; and 22 soldiers, while the Navy lost six civilian employees; three civilian contractors; and 33 sailors. Seven Defense Intelligence Agency (DIA) civilian employees died, and one Office of the Secretary of Defense (OSD) contractor. Lieutenant General Timothy Maude, an Army Deputy Chief of Staff, was the highest-ranking military official killed at the Pentagon.

Weeks after the attack, the death toll was estimated to be over 6,000, more than twice the number of deaths eventually confirmed. The city was only able to identify remains for about 1,600 of the World Trade Center victims. The medical examiner's office collected "about 10,000 unidentified bone and tissue fragments that cannot be matched to the list of the dead". Bone fragments were still being found in 2006 by workers who were preparing to demolish the damaged Deutsche Bank Building.

In 2010, a team of anthropologists and archaeologists searched for human remains and personal items at the Fresh Kills Landfill, where 72 more human remains were recovered, bringing the total found to 1,845. DNA profiling continues in an attempt to identify additional victims. In 2014, three coffin-size cases carrying 7,930 unidentified remains were transferred to a medical examiner's repository located at the same site as the National September 11 Memorial & Museum, but separated from the site by a wall. Victims' families are permitted to visit a private "reflection room" which is closed to the public. The choice to place the remains in an underground area attached to a museum has been controversial; families of some victims have attempted to have the remains instead interred in a separate, above-ground monument.

In July 2011, a team of scientists at the Office of Chief Medical Examiner continued efforts to identify remains, in the hope that improved technology will allow them to identify other victims. In August 2017, the 1,641st victim was identified as a result of newly available DNA technology, and a 1,642nd during July 2018. Three more victims were identified in October 2019, two in September 2021 and an additional two in September 2023. As of September 2023, 1,104 victims remain unidentified, amounting to 40% of the deaths in the World Trade Center attacks. On September 25, 2023, the FDNY reported that with the death of EMT Hilda Vannata and retired firefighter Robert Fulco, marking the 342nd and 343rd deaths from 9/11-related illnesses, the department had now lost the same number of firefighters, EMTs, and civilian members to 9/11-related illnesses as it did on the day of the attacks.

Damage
Along with the 110-floor Twin Towers, numerous other buildings at the World Trade Center site were destroyed or badly damaged, including WTC buildings 3   through    7 and St. Nicholas Greek Orthodox Church. The North Tower, South Tower, the Marriott Hotel (3   WTC), and 7    WTC were destroyed. The U.S. Customs House (6 World Trade Center), 4   World Trade Center, 5 World Trade Center, and both pedestrian bridges connecting buildings were severely damaged. The Deutsche Bank Building (still popularly referred to as the Bankers Trust Building) on 130 Liberty Street was partially damaged and demolished some years later, starting in 2007. The two buildings of the World Financial Center also suffered damage. The last fires at the World Trade Center site were extinguished on December 20, exactly 100 days after the attacks.

The Deutsche Bank Building across Liberty Street from the World Trade Center complex was later condemned as uninhabitable because of toxic conditions inside the office tower and was deconstructed. The Borough of Manhattan Community College's Fiterman Hall at 30 West Broadway was condemned due to extensive damage from the attacks, and then reopened in 2012.

Other neighbouring buildings (including 90 West Street and the Verizon Building) suffered major damage but have been restored. World Financial Center buildings, One Liberty Plaza, the Millenium Hilton, and 90 Church Street had moderate damage and have since been restored. Communications equipment on top of the North Tower was also destroyed, with only WCBS-TV maintaining a backup transmitter on the Empire State Building, but media stations were quickly able to reroute the signals and resume their broadcasts. The PATH train system's World Trade Center station was located under the complex. As a result, the station was demolished when the towers collapsed, and the tunnels leading to Exchange Place station in Jersey City, New Jersey, were flooded with water. The station was rebuilt as the $4   billion World Trade Center Transportation Hub, which reopened in March 2015. The Cortlandt Street station on the New York City Subway's IRT Broadway–Seventh Avenue Line was also in close proximity to the World Trade Center complex, and the entire station, along with the surrounding track, was reduced to rubble. The latter station was rebuilt and reopened to the public on September 8, 2018. The Pentagon was extensively damaged by the impact of American Airlines Flight 77 and the ensuing fires, causing one section of the building to collapse. As the plane approached the Pentagon, its wings knocked down light poles and its right engine hit a power generator before crashing into the western side of the building. The plane hit the Pentagon at the first-floor level. The front part of the fuselage disintegrated on impact, while the mid and tail sections kept moving for another fraction of a second. Debris from the tail section penetrated the furthest into the building, breaking through 310 ft of the three outermost of the building's five rings.

Rescue efforts
The New York City Fire Department (FDNY) deployed more than 200 units (approximately half of the department) to the World Trade Center. Their efforts were supplemented by numerous off-duty firefighters and emergency medical technicians. The New York City Police Department (NYPD) sent its Emergency Service Units and other police personnel and deployed its aviation unit. The NYPD aviation unit assessed the situation and decided that helicopter rescues from the towers were not feasible. Numerous police officers of the Port Authority Police Department (PAPD) also participated in rescue efforts. Once on the scene, the FDNY, the NYPD, and the PAPD did not coordinate efforts and performed redundant searches for civilians.

As conditions deteriorated, the NYPD aviation unit relayed information to police commanders, who issued orders for personnel to evacuate the towers; most NYPD officers were able to safely evacuate before the buildings collapsed. With separate command posts set up and incompatible radio communications between the agencies, warnings were not passed along to FDNY commanders.

After the first tower collapsed, FDNY commanders issued evacuation warnings. Due to technical difficulties with malfunctioning radio repeater systems, many firefighters never heard the evacuation orders. 9-1-1 dispatchers also received information from callers that was not passed along to commanders on the scene.

Reactions
The 9/11 attacks resulted in immediate responses to the event, including domestic reactions; closings and cancellations; hate crimes; Muslim-American responses to the event; international responses to the attack; and military responses to the events. Shortly after the attacks, a U.S. government fund that was created by an Act of Congress named the September 11th Victim Compensation Fund. The purpose of the fund was to compensate the victims of the attacks and their families with the quid pro quo of their agreement not to file lawsuits against the airline corporations involved. Legislation authorizes the fund to disburse a maximum of $7.375 billion, including operational and administrative costs, of U.S. government funds. The fund was set to expire by 2020 but was in 2019 prolonged to allow claims to be filed until October 2090.

Immediate response
At 8:32 a.m., FAA officials were notified Flight   11 had been hijacked and they, in turn, notified the North American Aerospace Defense Command (NORAD). NORAD scrambled two F-15s from Otis Air National Guard Base in Massachusetts and they were airborne by 8:53 a.m. Because of slow and confused communication from FAA officials, NORAD had nine minutes' notice, and no notice about any of the other flights before they crashed.

After both of the Twin Towers had already been hit, more fighters were scrambled from Langley Air Force Base in Virginia at 9:30 a.m. At 10:20 a.m., Vice President Dick Cheney issued orders to shoot down any commercial aircraft that could be positively identified as being hijacked. These instructions were not relayed in time for the fighters to take action. Some fighters took to the air without live ammunition, knowing that to prevent the hijackers from striking their intended targets, the pilots might have to intercept and crash their fighters into the hijacked planes, possibly ejecting at the last moment.

For the first time in U.S. history, the emergency preparedness plan called Security Control of Air Traffic and Air Navigation Aids (SCATANA) was invoked, thus stranding tens of thousands of passengers across the world. Ben Sliney, in his first day as the National Operations Manager of the FAA, ordered that American airspace would be closed to all international flights, causing about 500 flights to be turned back or redirected to other countries. Canada received 226 of the diverted flights and launched Operation Yellow Ribbon to deal with the large numbers of grounded planes and stranded passengers.

The 9/11 attacks had immediate effects on the American people. Police and rescue workers from around the country took a leave of absence from their jobs and travelled to New York City to help recover bodies from the twisted remnants of the Twin Towers. Blood donations across the U.S. surged in the weeks after 9/11.

The deaths of adults in the attacks resulted in over 3,000 children losing a parent. Subsequent studies documented children's reactions to these actual losses and feared losses of life, the protective environment in the attacks' aftermath, and the effects on surviving caregivers.

Domestic reactions
Following the attacks, President George W. Bush's approval rating increased to 90%. On September 20, 2001, he addressed the nation and a joint session of Congress regarding the events of September 11 and the subsequent nine days of rescue and recovery efforts, and described his intended response to the attacks. New York City mayor Rudy Giuliani's highly visible role resulted in praise in New York and nationally.

Many relief funds were immediately set up to assist the attacks' victims, with the task of providing financial assistance to the survivors of the attacks and the victims' families. By the deadline for victims' compensation on September 11, 2003, 2,833 applications had been received from the families of those who were killed.

Contingency plans for the continuity of government and the evacuation of leaders were implemented soon after the attacks. Congress was not told that the United States had been under a continuity of government status until February 2002.

In the largest restructuring of the U.S. government in contemporary history, the United States enacted the Homeland Security Act of 2002, creating the U.S. Department of Homeland Security. Congress also passed the USA PATRIOT Act, saying it would help detect and prosecute terrorism and other crimes. Civil liberties groups have criticized the PATRIOT Act, saying it allows law enforcement to invade citizens' privacy and that it eliminates judicial oversight of law enforcement and domestic intelligence.

To effectively combat future acts of terrorism, the National Security Agency (NSA) was given broad powers. NSA commenced warrantless surveillance of telecommunications, which was sometimes criticized as permitting the agency "to eavesdrop on telephone and e-mail communications between the United States and people overseas without a warrant". In response to requests by various intelligence agencies, the United States Foreign Intelligence Surveillance Court permitted an expansion of powers by the U.S. government in seeking, obtaining, and sharing information on U.S. citizens as well as non-U.S. people from around the world.

Hate crimes
Six days after the attacks, President Bush made a public appearance at Washington, D.C.'s largest Islamic Center and acknowledged the "incredibly valuable contribution" that millions of American Muslims made to their country and called for them "to be treated with respect". Numerous incidents of harassment and hate crimes against Muslims and South Asians were reported in the days following the attacks.

Sikhs were also subject to targeting due to the use of turbans in the Sikh faith, which are stereotypically associated with Muslims. There were reports of attacks on mosques and other religious buildings (including the firebombing of a Hindu temple), and assaults on individuals, including one murder: Balbir Singh Sodhi, a Sikh mistaken for a Muslim, who was fatally shot on September 15, 2001, in Mesa, Arizona. Two dozen members of Osama bin Laden's family were urgently evacuated out of the country on a private charter plane under FBI supervision three days after the attacks.

According to an academic study, people perceived to be Middle Eastern were as likely to be victims of hate crimes as followers of Islam during this time. The study also found a similar increase in hate crimes against people who may have been perceived as Muslims, Arabs, and others thought to be of Middle Eastern origin. A report by the South Asian American advocacy group known as South Asian Americans Leading Together documented media coverage of 645 bias incidents against Americans of South Asian or Middle Eastern descent between September 11 and 17, 2001. Various crimes such as vandalism, arson, assault, shootings, harassment, and threats in numerous places were documented. Women wearing hijab were also targeted.

Discrimination and racial profiling
A poll of Arab-Americans, conducted in May 2002, found that 20% had personally experienced discrimination since September 11. A July 2002 poll of Muslim Americans found that 48% believed their lives had changed for the worse since September 11, and 57% had experienced an act of bias or discrimination.

Following the September 11 attacks, many Pakistani Americans identified themselves as Indians to avoid potential discrimination and obtain jobs (Pakistan was created as a result of the partition of India in 1947).

By May 2002, there were 488 complaints of employment discrimination reported to the U.S. Equal Employment Opportunity Commission (EEOC). 301 of those were complaints from people fired from their jobs. Similarly, by June 2002, the U.S. Department of Transportation (DOT) had investigated 111 September 11th-related complaints from airline passengers purporting that their religious or ethnic appearance caused them to be singled out at security screenings. DOT investigated an additional 31 complaints from people who alleged they were completely blocked from boarding aeroplanes on the same grounds.

Muslim American response
Muslim organizations in the United States were swift to condemn the attacks and called "upon Muslim Americans to come forward with their skills and resources to help alleviate the sufferings of the affected people and their families". These organizations included the Islamic Society of North America, American Muslim Alliance, American Muslim Council, Council on American-Islamic Relations, Islamic Circle of North America, and the Shari'a Scholars Association of North America. Along with monetary donations, many Islamic organizations launched blood drives and provided medical assistance, food, and shelter for victims.

Interfaith efforts
Curiosity about Islam increased after the attacks. As a result, many mosques and Islamic centres began holding open houses and participating in outreach efforts to educate non-Muslims about the faith. In the first 10 years after the attacks, interfaith community service increased from 8 to 20 percent and the percentage of U.S. congregations involved in interfaith worship doubled from 7 to 14 percent.

International reactions
The attacks were denounced by mass media and governments worldwide. Across the globe, nations offered pro-American support and solidarity. Leaders in most Middle Eastern countries, as well as Libya and Afghanistan, condemned the attacks. Iraq was a notable exception, with an immediate official statement that "the American cowboys are reaping the fruit of their crimes against humanity". The government of Saudi Arabia officially condemned the attacks, but privately many Saudis favored bin Laden's cause.

Although Palestinian Authority (PA) president Yasser Arafat also condemned the attacks, there were reports of celebrations of disputed size in the West Bank, Gaza Strip, and East Jerusalem. Palestinian leaders discredited news broadcasters that justified the attacks or showed celebrations, and the Authority claimed such celebrations do not represent the Palestinians' sentiment, adding that it would not allow "a few kids" to "smear the real face of the Palestinians". Footage by CNN and other news outlets were suggested by a report originating at a Brazilian university to be from 1991; this was later proven to be a false accusation, resulting in a statement being issued by CNN. As in the United States, the aftermath of the attacks saw tensions increase in other countries between Muslims and non-Muslims.

United Nations Security Council Resolution 1368 condemned the attacks and expressed readiness to take all necessary steps to respond and combat all forms of terrorism in accordance with their Charter. Numerous countries introduced anti-terrorism legislation and froze bank accounts they suspected of Al-Qaeda ties. Law enforcement and intelligence agencies in a number of countries arrested alleged terrorists.

British Prime Minister Tony Blair said Britain stood "shoulder to shoulder" with the United States. A few days later, Blair flew to Washington, D.C., to affirm British solidarity with the United States. In a speech to Congress nine days after the attacks, which Blair attended as a guest, President Bush declared "America has no truer friend than Great Britain". Subsequently, Prime Minister Blair embarked on two months of diplomacy to rally international support for military action; he held 54 meetings with world leaders and travelled more than 40,000 miles (60,000   km).

The U.S. set up the Guantanamo Bay detention camp to hold inmates they defined as "illegal enemy combatants". The legitimacy of these detentions has been questioned by the European Union and human rights organizations.

On September 25, 2001, Iran's fifth president, Mohammad Khatami, meeting British Foreign Secretary Jack Straw, said: "Iran fully understands the feelings of the Americans about the terrorist attacks in New York and Washington on September 11". He said although the American administrations had been at best indifferent about terrorist operations in Iran (since 1979), the Iranians felt differently and had expressed their sympathetic feelings with bereaved Americans in the tragic incidents in the two cities. He also stated that "Nations should not be punished in place of terrorists".

According to Radio Farda's website, when the news of the attacks was released, some Iranian citizens gathered in front of the Embassy of Switzerland in Tehran, which serves as the protecting power of the United States in Iran (U.S. interests-protecting office in Iran), to express their sympathy, and some of them lit candles as a symbol of mourning. This piece of news on Radio Farda's website also states that in 2011, on the anniversary of the attacks, the United States Department of State published a post on its blog, in which the Department thanked the Iranian people for their sympathy and stated that it would never forget Iranian people's kindness on those harsh days. After the attacks, both the President and the Supreme Leader of Iran, condemned the attacks. The BBC and Time magazine published reports on holding candlelit vigils for the victims of Iranian citizens on their websites. According to Politico Magazine, following the attacks, Ali Khamenei, the Supreme Leader of Iran, "suspended the usual 'Death to America' chants at Friday prayers" temporarily.

In September 2001, shortly after the attacks, some fans of AEK Athens burned an Israeli flag and unsuccessfully tried to burn an American flag. Though the American flag did not catch fire, the fans booed during a moment of silence for victims of the attacks.

Military operations
At 2:40 p.m. on September 11, Secretary of Defense Donald Rumsfeld was issuing rapid orders to his aides to look for evidence of Iraqi involvement. According to notes taken by senior policy official Stephen Cambone, Rumsfeld asked for, "Best info fast. Judge whether they are good enough to hit S.H. [Saddam Hussein] at the same time. Not only UBL" [Osama bin Laden]. Cambone's notes quoted Rumsfeld as saying, "Need to move swiftly – Near term target needs – go massive – sweep it all up. Things related and not".

In a meeting at Camp David on September 15 the Bush administration rejected the idea of attacking Iraq in response to the September 11 attacks. Nonetheless, they later invaded the country with allies, citing "Saddam Hussein's support for terrorism". At the time, as many as seven in ten Americans believed the Iraqi president played a role in the 9/11 attacks. Three years later, Bush conceded that he had not.

The NATO council declared that the terrorist attacks on the United States were an attack on all NATO nations that satisfied Article 5 of the NATO charter. This marked the first invocation of Article 5, which had been written during the Cold War with an attack by the Soviet Union in mind. Australian Prime Minister John Howard, who was in Washington, D.C., during the attacks, invoked Article IV of the ANZUS treaty. The Bush administration announced a war on terror, with the stated goals of bringing bin Laden and Al-Qaeda to justice and preventing the emergence of other terrorist networks. These goals would be accomplished by imposing economic and military sanctions against states harbouring terrorists, and increasing global surveillance and intelligence sharing.

On September 14, 2001, the U.S. Congress passed the Authorization for the use of Military Force Against Terrorists, which grants the President the authority to use all "necessary and appropriate force" against those whom he determined "planned, authorized, committed or aided" the September 11 attacks or who harboured said persons or groups. It is still in effect to this day.

On October 7, 2001, the War in Afghanistan began when U.S. and British forces initiated aerial bombing campaigns targeting Taliban and Al-Qaeda camps, then later invaded Afghanistan with ground troops of the Special Forces. This eventually led to the overthrow of the Taliban's rule of Afghanistan with the Fall of Kandahar on December 7, 2001, by U.S.-led coalition forces.

Al-Qaeda leader Osama bin Laden, who went into hiding in the White Mountains, was targeted by U.S. coalition forces in the Battle of Tora Bora, but he escaped across the Pakistani border and remained out of sight for almost ten years. In an interview with Tayseer Allouni on October 21, 2001, Bin Laden stated: "'The events proved the extent of terrorism that America exercises in the world. Bush stated that the world has to be divided in two: Bush and his supporters, and any country that doesn't get into the global crusade is with the terrorists. What terrorism is clearer than this? Many governments were forced to support this 'new terrorism.'.. America wouldn't live in security until we live it truly in Palestine. This showed the reality of America, which puts Israel's interest above its own people's interest. America won't get out of this crisis until it gets out of the Arabian Peninsula, and until it stops its support of Israel."

The Philippines and Indonesia, among other nations with their internal conflicts with Islamic terrorism, also increased their military readiness. The military forces of the United States of America and the Islamic Republic of Iran cooperated to overthrow the Taliban regime, which had had conflicts with the government of Iran. Iran's Quds Force helped U.S. forces and Afghan rebels in the 2001 uprising in Herat.

Health issues
Hundreds of thousands of tons of toxic debris containing more than 2,500 contaminants and known carcinogens were spread across Lower Manhattan when the Twin Towers' collapsed. Exposure to the toxins in the debris is alleged to have contributed to fatal or debilitating illnesses among people who were at Ground Zero. The Bush administration ordered the Environmental Protection Agency (EPA) to issue reassuring statements regarding air quality in the aftermath of the attacks, citing national security, but the EPA did not determine that air quality had returned to pre-September 11 levels until June 2002.

Health effects extended to residents, students, and office workers of Lower Manhattan and nearby Chinatown. Several deaths have been linked to the toxic dust, and victims' names were included in the World Trade Center memorial. Approximately 18,000 people have been estimated to have developed illnesses as a result of the toxic dust. There is also scientific speculation that exposure to various toxic products in the air may have negative effects on fetal development. A study of rescue workers released in April 2010 found that all those studied had impaired lung functions and that 30–40% were reporting little or no improvement in persistent symptoms that started within the first year of the attack.

Years after the attacks, legal disputes over the costs of illnesses related to the attacks were still in the court system. On October 17, 2006, a federal judge rejected New York City's refusal to pay for health costs for rescue workers, allowing for the possibility of numerous suits against the city. Government officials have been faulted for urging the public to return to lower Manhattan in the weeks shortly after the attacks. Christine Todd Whitman, administrator of the EPA in the attacks' aftermath, was heavily criticized by a U.S. District Judge for incorrectly saying that the area was environmentally safe. Mayor Giuliani was criticized for urging financial industry personnel to return quickly to the greater Wall Street area.

On December 22, 2010, the United States Congress passed the James L. Zadroga 9/11 Health and Compensation Act, which President Barack Obama signed into law on January 2, 2011. It allocated $4.2   billion to create the World Trade Center Health Program, which provides testing and treatment for people suffering from long-term health problems related to the 9/11 attacks. The WTC Health Program replaced preexisting 9/11-related health programs such as the Medical Monitoring and Treatment Program and the WTC Environmental Health Center program.

In 2020, the NYPD confirmed that 247 NYPD police officers had died due to 9/11-related illnesses. In September 2022, the FDNY confirmed that the total number of firefighters who died due to 9/11-related illnesses was 299. Both agencies believe that the death toll will rise dramatically in the coming years. The Port Authority of New York and New Jersey Police Department (PAPD), the law enforcement agency with jurisdiction over the World Trade Center due to the Port Authority of New York and New Jersey owning the site confirmed that four of its police officers have died of 9/11-related illnesses. The chief of the PAPD at the time, Joseph Morris, made sure that industrial-grade respirators were provided to all PAPD police officers within 48 hours and decided that the same 30 to 40 police officers would be stationed at the World Trade Center pile, drastically lowering the number of total PAPD personnel who would be exposed to the air. The FDNY and NYPD had rotated hundreds, if not thousands, of different personnel from all over New York City to the pile, which exposed many of them to dust that would give them cancer or other diseases years or decades later. Also, they were not given adequate respirators and breathing equipment that could have prevented future diseases.

Economic
The attacks had a significant economic impact on the United States and world markets. The stock exchanges did not open on September 11 and remained closed until September 17. Reopening, the Dow Jones Industrial Average (DJIA) fell 684 points, or 7.1%, to 8921, a record-setting one-day point decline. By the end of the week, the DJIA had fallen 1,369.7 points (14.3%), at the time its largest one-week point drop in history. In 2001 dollars, U.S. stocks lost $1.4   trillion in valuation for the week.

In New York City, about 430,000 job months and $2.8   billion in wages were lost in the first three months after the attacks. The economic effects were mainly on the economy's export sectors. The city's GDP was estimated to have declined by $27.3   billion for the last three months of 2001 and all of 2002. The U.S. government provided $11.2   billion in immediate assistance to the Government of New York City in September 2001, and $10.5    billion in early 2002 for economic development and infrastructure needs.

Also hurt were small businesses in Lower Manhattan near the World Trade Center (18,000 of which were destroyed or displaced), resulting in lost jobs and wages. Assistance was provided by Small Business Administration loans; federal government Community Development Block Grants; and Economic Injury Disaster Loans. Some 31900000 sqft of Lower Manhattan office space was damaged or destroyed. Many wondered whether these jobs would return, and if the damaged tax base would recover. Studies of 9/11's economic effects show the Manhattan office real-estate market and office employment were less affected than first feared, because of the financial services industry's need for face-to-face interaction.

North American air space was closed for several days after the attacks and air travel decreased upon its reopening, leading to a nearly 20% cutback in air travel capacity, and exacerbating financial problems in the struggling U.S. airline industry.

The September 11 attacks also led to the U.S. wars in Afghanistan and Iraq, as well as additional homeland security spending, totaling at least $5   trillion.

Effects in Afghanistan
Most of the Afghan population was already going hungry at the time of the September 11 attacks. In the aftermath of the attacks, tens of thousands of people attempted to flee Afghanistan due to the possibility of military retaliation by the United States. Pakistan, already home to many Afghan refugees from previous conflicts, closed its border with Afghanistan on September 17, 2001. Thousands of Afghans also fled to the frontier with Tajikistan, although were denied entry. The Taliban leaders in Afghanistan themselves pleaded against military action, saying "We appeal to the United States not to put Afghanistan into more misery because our people have suffered so much", referring to two decades of conflict and the humanitarian crisis attached to it.

All United Nations expatriates had left Afghanistan after the attacks and no national or international aid workers were at their post. Workers were instead preparing in bordering countries like Pakistan, China and Uzbekistan to prevent a potential "humanitarian catastrophe", amid a critically low food stock for the Afghan population. The World Food Programme stopped importing wheat to Afghanistan on September 12 due to security risks. The Wall Street Journal suggested the creation of a buffer zone in an inevitable war, similarly as in the Bosnian War.

Approximately one month after the attacks, the United States led a broad coalition of international forces to overthrow the Taliban regime from Afghanistan for their harboring of Al-Qaeda. Though Pakistani authorities were initially reluctant to align themselves with the United States against the Taliban, they permitted the coalition access to their military bases, and arrested and handed over to the U.S. over 600 suspected Al-Qaeda members.

In a speech by the Nizari Ismaili Imam at the Nobel Institute in 2005, Aga Khan IV stated that the "9/11 attack on the United States was a direct consequence of the international community ignoring the human tragedy that was Afghanistan at that time".

In 2011, the U.S. and NATO under President Obama initiated a drawdown of troops in Afghanistan finalized in 2016. During the presidencies of Donald Trump and Joe Biden in 2020 and 2021, the United States alongside its NATO allies withdrew all troops from Afghanistan completing the withdrawal of all regular U.S. troops on August 30, 2021, 12 days before the 20th anniversary of the September 11 attacks,  The withdrawal marked the end of the 2001–2021 War in Afghanistan. Biden said that after nearly 20 years of war, it was clear that the U.S. military could not transform Afghanistan into a modern democracy.

The second emir of Al-Qaeda, Ayman al-Zawahiri, a close associate of bin Laden, was killed in a U.S. drone strike at his home in Kabul, Afghanistan on July 31, 2022.

Cultural influence
The impact of 9/11 extends beyond geopolitics and into society and culture in general. Immediate responses to 9/11 included greater focus on home life and time spent with family, higher church attendance, and increased expressions of patriotism such as the flying of American flags. The radio industry responded by removing certain songs from playlists, and the attacks have subsequently been used as background, narrative, or thematic elements in film, music, literature, and humour. Already-running television shows as well as programs developed after 9/11 have reflected post-9/11 cultural concerns.

9/11 conspiracy theories have become a social phenomenon, despite a lack of support from expert scientists, engineers, and historians. 9/11 has also had a major impact on the religious faith of many individuals; for some it strengthened, to find consolation to cope with the loss of loved ones and overcome their grief; others started to question their faith or lose it entirely because they could not reconcile it with their view of religion.

The culture of America, after the attacks, is noted for heightened security and an increased demand thereof, as well as paranoia and anxiety regarding future terrorist attacks against most of the nation. Psychologists have also confirmed that there has been an increased amount of national anxiety in commercial air travel. Anti-Muslim hate crimes rose nearly ten-fold in 2001 and have subsequently remained "roughly five times higher than the pre-9/11 rate".

Government policies towards terrorism
As a result of the attacks, many governments across the world passed legislation to combat terrorism. In Germany, where several of the 9/11 terrorists had resided and taken advantage of that country's liberal asylum policies, two major anti-terrorism packages were enacted. The first removed legal loopholes that permitted terrorists to live and raise money in Germany. The second addressed the effectiveness and communication of intelligence and law enforcement. Canada passed the Canadian Anti-Terrorism Act, their first anti-terrorism law. The United Kingdom passed the Anti-terrorism, Crime and Security Act 2001 and the Prevention of Terrorism Act 2005. New Zealand enacted the Terrorism Suppression Act 2002.

In the United States, the Department of Homeland Security was created by the Homeland Security Act of 2002 to coordinate domestic anti-terrorism efforts. The USA Patriot Act gave the federal government greater powers, including the authority to detain foreign terror suspects for a week without charge; to monitor terror suspects' telephone communications, e-mail, and Internet use; and to prosecute suspected terrorists without time restrictions. The FAA ordered that aeroplane cockpits be reinforced to prevent terrorists from gaining control of planes and assigned sky marshals to flights.

Further, the Aviation and Transportation Security Act made the federal government, rather than airports, responsible for airport security. The law created the Transportation Security Administration to inspect passengers and luggage, causing long delays and concern over passenger privacy. After suspected abuses of the USA Patriot Act were brought to light in June 2013 with articles about the collection of American call records by the NSA and the PRISM program, Representative Jim Sensenbrenner (of Wisconsin), who introduced the Patriot Act in 2001, said that the NSA overstepped its bounds.

Criticism of the war on terror has focused on its morality, efficiency, and cost. According to a 2021 report by the Costs of War project, the several post-9/11 wars participated in by the United States in its War on Terror have caused the displacement, conservatively calculated, of 38 million people in Afghanistan, Pakistan, Iraq, Libya, Syria, Yemen, Somalia, and the Philippines. They estimated these wars caused the deaths of 897,000 to 929,000 people directly and cost $8 trillion. In a 2023 report, the Costs of War project estimated that there have been between 3.6 and 3.7 million indirect deaths in the post-9/11 war zones, with the total death toll being 4.5 to 4.6 million. The report defined post-9/11 war zones as conflicts that included significant United States counter-terrorism operations since 9/11, which in addition to the wars in Iraq, Afghanistan and Pakistan, also includes the civil wars in Syria, Yemen, Libya and Somalia. The report derived its estimate of indirect deaths using a calculation from the Geneva Declaration of Secretariat which estimates that for every person directly killed by war, four more die from the indirect consequences of war. The report's author Stephanie Savell stated that in an ideal scenario, the preferable way of quantifying the total death toll would have been by studying excess mortality, or by using on-the-ground researchers in the affected countries. The U.S. Constitution and U.S. law prohibits the use of torture, yet such human rights violations occurred during the War on Terror under the euphemism "enhanced interrogation". In 2005, The Washington Post and Human Rights Watch (HRW) published revelations concerning CIA flights and "black sites", covert prisons operated by the CIA. The term "torture by proxy" is used by some critics to describe situations in which the CIA and other U.S. agencies have transferred suspected terrorists to countries known to employ torture.

Legal proceedings
As all 19 hijackers died in the attacks, they were never prosecuted. Osama bin Laden was never formally indicted but was after a 10-year manhunt killed by U.S. special forces on May 2, 2011, in his compound in Abbottabad, Pakistan. The main trial of the attacks against Mohammed and his co-conspirators Walid bin Attash, Ramzi bin al-Shibh, Ammar al-Baluchi, and Mustafa Ahmad al Hawsawi remains unresolved. Khalid Sheikh Mohammed was arrested on March 1, 2003, in Rawalpindi, Pakistan, by Pakistani security officials working with the CIA. He was then held at multiple CIA secret prisons and Guantanamo Bay, where he was interrogated and tortured with methods including waterboarding. In 2003, Mustafa al-Hawsawi and Abd al-Aziz Ali were arrested and transferred to U.S. custody. Both would later be accused of providing money and travel assistance to the hijackers. During U.S. hearings at Guantanamo Bay in March 2007, Mohammed again confessed his responsibility for the attacks, stating he "was responsible for the 9/11 operation from A to Z" and that his statement was not made under duress. In January 2023, the US government opened up about a potential plea deal, with Biden giving up on the effort in September that year.

To date, only peripheral persons have thus been convicted for charges in connection with the attacks. These include:


 * Zacarias Moussaoui who was indicted in December 2001 and sentenced to life in prison without the possibility of parole in May 2006 by a U.S. federal jury
 * Mounir El Motassadeq who was first convicted in February 2003 by a Federal Court of Justice in Germany and was deported to Morocco in October 2018 after serving his sentence
 * Abu Dahdah who was arrested in November 2001, sentenced by a Spanish High Court and released from prison in May 2013.

FBI
Immediately after the attacks, the Federal Bureau of Investigation started PENTTBOM, the largest criminal inquiry in United States history. At its height, more than half of the FBI's agents worked on the investigation and followed a half-million leads. The FBI concluded that there was "clear and irrefutable" evidence linking Al-Qaeda and bin Laden to the attacks.

The FBI quickly identified the hijackers, including leader Mohamed Atta, when his luggage was discovered at Boston's Logan Airport. Atta had been forced to check two of his three bags due to space limitations on the 19-seat commuter flight he took to Boston. Due to a new policy instituted to prevent flight delays, the luggage failed to make it aboard American Airlines Flight 11 as planned. The luggage contained the hijackers' names, assignments, and Al-Qaeda connections. "It had all these Arab-language [sic] papers that amounted to the Rosetta stone of the investigation", said one FBI agent. Within hours of the attacks, the FBI released the names and in many cases the personal details of the suspected pilots and hijackers. Abu Jandal, who served as bin Laden's chief bodyguard for years, confirmed the identity of seven hijackers as Al-Qaeda members during interrogations with the FBI on September 17. He had been jailed in a Yemeni prison since 2000. On September 27, 2001, photos of all 19 hijackers were released, along with information about possible nationalities and aliases. Fifteen of the men were from Saudi Arabia, two were from the United Arab Emirates, one was from Egypt, and one was from Lebanon.

By midday, the U.S. National Security Agency and German intelligence agencies had intercepted communications pointing to Osama bin Laden. Two of the hijackers were known to have traveled with a bin Laden associate to Malaysia in 2000 and hijacker Mohamed Atta had previously gone to Afghanistan. He and others were part of a terrorist cell in Hamburg. One of the members of the Hamburg cell in Germany was discovered to have been in communication with Khalid Sheikh Mohammed who was identified as a member of Al-Qaeda.

Authorities in the United States and the United Kingdom also obtained electronic intercepts, including telephone conversations and electronic bank transfers, which indicated that Mohammed Atef, a bin Laden deputy, was a key figure in the planning of the 9/11 attacks. Intercepts were also obtained that revealed conversations that took place days before September 11 between bin Laden and an associate in Pakistan. In those conversations, the two referred to "an incident that would take place in America on, or around, September 11" and they discussed potential repercussions. In another conversation with an associate in Afghanistan, bin Laden discussed the "scale and effects of a forthcoming operation". These conversations did not specifically mention the World Trade Center, the Pentagon, or other specifics.

In their annual violent crime index for the year 2001, the FBI recorded the deaths from the attacks as murder, in separate tables so as not to mix them with other reported crimes for that year. In a disclaimer, the FBI stated that "the number of deaths is so great that combining it with the traditional crime statistics will have an outlier effect that falsely skews all types of measurements in the program's analyses". New York City also did not include the deaths in their annual crime statistics for 2001.

CIA
In 2004, John L. Helgerson, the Inspector General of the Central Intelligence Agency (CIA), conducted an internal review of the agency's pre-9/11 performance and was harshly critical of senior CIA officials for not doing everything possible to confront terrorism. According to Philip Giraldi in The American Conservative, Helgerson criticized their failure to stop two of the 9/11 hijackers, Nawaf al-Hazmi and Khalid al-Mihdhar, as they entered the United States and their failure to share information on the two men with the FBI.

In May 2007, senators from both major U.S. political parties (the Republican and Democratic party) drafted legislation to make the review public. One of the backers, Senator Ron Wyden said, "The American people have a right to know what the Central Intelligence Agency was doing in those critical months before 9/11". The report was released in 2009 by President Barack Obama.

Congressional inquiry
In February 2002, the Senate Select Committee on Intelligence and the House Permanent Select Committee on Intelligence formed a joint inquiry into the performance of the U.S. Intelligence Community. Their 832-page report released in December 2002 detailed failings of the FBI and CIA to use available information, including about terrorists the CIA knew were in the United States, to disrupt the plots. The joint inquiry developed its information about possible involvement of Saudi Arabian government officials from non-classified sources. Nevertheless, the Bush administration demanded 28 related pages remain classified. In December 2002, the inquiry's chair Bob Graham (D-FL) revealed in an interview that there was "evidence that there were foreign governments involved in facilitating the activities of at least some of the terrorists in the United States". September 11 victim families were frustrated by the unanswered questions and redacted material from the congressional inquiry and demanded an independent commission. September 11 victim families, members of Congress and the Saudi Arabian government are still seeking the release of the documents. In June 2016, CIA chief John Brennan said that he believes 28 redacted pages of a congressional inquiry into 9/11 will soon be made public, and that they will prove that the government of Saudi Arabia had no involvement in the September 11 attacks.

In September 2016, Congress passed the Justice Against Sponsors of Terrorism Act that would allow relatives of victims of the September 11 attacks to sue Saudi Arabia for its government's alleged role in the attacks.

9/11 Commission
The National Commission on Terrorist Attacks Upon the United States, popularly known as the 9/11 Commission, chaired by Thomas Kean, governor of New Jersey from 1982 to 1990, was formed in late 2002 to prepare a thorough account of the circumstances surrounding the attacks, including preparedness for and the immediate response to the attacks. On July 22, 2004, the commission issued the 9/11 Commission Report, a 585-page report based on its investigations and interviews. The report detailed the events leading up to the September 11 attacks, concluding that they were carried out by Al-Qaeda. The commission also examined how security and intelligence agencies were inadequately coordinated to prevent the attacks.

According to the report, "We believe the 9/11 attacks revealed four kinds of failures: in imagination, policy, capabilities, and management". The commission made numerous recommendations on how to prevent future attacks, and in 2011 was dismayed that several of its recommendations had yet to be implemented.

National Institute of Standards and Technology
The U.S. National Institute of Standards and Technology (NIST) investigated the collapses of the Twin Towers and 7   WTC. The investigations examined why the buildings collapsed and what fire protection measures were in place, and evaluated how fire protection systems might be improved in future construction. The investigation into the collapse of 1   WTC and 2    WTC was concluded in October 2005 and that of 7    WTC was completed in August 2008.

NIST found that the fireproofing on the Twin Towers' steel infrastructures was blown off by the initial impact of the planes and that had this not occurred, the towers likely would have remained standing. A 2007 study of the north tower's collapse published by researchers of Purdue University determined that since the plane's impact had stripped off much of the structure's thermal insulation, the heat from a typical office fire would have softened and weakened the exposed girders and columns enough to initiate the collapse regardless of the number of columns cut or damaged by the impact.

The director of the original investigation stated that "the towers did amazingly well. The terrorist aircraft didn't bring the buildings down; it was the fire that followed. It was proven that you could take out two-thirds of the columns in a tower and the building would still stand". The fires weakened the trusses supporting the floors, making the floors sag. The sagging floors pulled on the exterior steel columns causing the exterior columns to bow inward.

With the damage to the core columns, the buckling exterior columns could no longer support the buildings, causing them to collapse. Additionally, the report found the towers' stairwells were not adequately reinforced to provide adequate emergency escape for people above the impact zones. NIST concluded that uncontrolled fires in 7   WTC caused floor beams and girders to heat and subsequently "caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down".

Alleged Saudi government role
In July 2016, the Obama administration released a document compiled by U.S. investigators Dana Lesemann and Michael Jacobson, known as "File 17", which contains a list naming three dozen people, including the suspected Saudi intelligence officers attached to Saudi Arabia's embassy in Washington, D.C., which connects Saudi Arabia to the hijackers.

In September 2016, Congress passed the Justice Against Sponsors of Terrorism Act. The practical effect of the legislation was to allow the continuation of a longstanding civil lawsuit brought by families of victims of the September 11 attacks against Saudi Arabia for its government's alleged role in the attacks. In March 2018, a U.S. judge formally allowed a suit to move forward against the government of Saudi Arabia brought by 9/11 survivors and victims' families.

In 2022, the families of some 9/11 victims obtained two videos and a notepad seized from Saudi national Omar al-Bayoumi by the British courts. The first video showed him hosting a party in San Diego for Nawaf al-Hazmi and Khalid al-Mihdhar, the first two hijackers to arrive in the U.S. The other video showed al-Bayoumi greeting the cleric Anwar al-Awlaki, who was blamed for radicalizing Americans and later killed in a CIA drone strike. The notepad depicted a hand-drawn aeroplane and some mathematical equations that, according to a pilot's court statement, might have been used to calculate the rate of descent to get to a target. According to a 2017 FBI memo, from the late 1990s up until the 9/11 attack, al-Bayoumi was a paid cooptee of the Saudi General Intelligence Presidency. he is believed to be living in Saudi Arabia, which has denied any involvement in 9/11.

Reconstruction
On the day of the attacks, New York City mayor Rudy Giuliani stated: "We will rebuild. We're going to come out of this stronger than before, politically stronger, economically stronger. The skyline will be made whole again".

Within hours of the attack, a substantial search and rescue operation was launched. After months of around-the-clock operations, the World Trade Center site was cleared by the end of May 2002. The damaged section of the Pentagon was rebuilt and occupied within a year of the attacks. The temporary World Trade Center PATH station opened in late 2003 and construction of the new 7   World Trade Center was completed in 2006. Work on rebuilding the main World Trade Center site was delayed until late 2006 when leaseholder Larry Silverstein and the Port Authority of New York and New Jersey agreed on financing. The construction of One World Trade Center began on April 27, 2006, and reached its full height on May 20, 2013. The spire was installed atop the building at that date, putting One   WTC's height at 1,776 feet (541    m) and thus claiming the title of the tallest building in the Western Hemisphere. One WTC finished construction and opened on November 3, 2014.

On the World Trade Center site, three more office towers were to be built one block east of where the original towers stood. 4   WTC, meanwhile, opened in November 2013, making it the second tower on the site to open behind 7    World Trade Center, as well as the first building on the Port Authority property. 3   WTC opened on June 11, 2018, becoming the fourth skyscraper at the site to be completed. In December 2022, the Nicholas Greek Orthodox Church fully reopened for regular services followed by the opening of the Ronald O. Perelman Performing Arts Center in September 2023. With construction beginning in 2008, 2 World Trade Center remains as of 2023 unfinished. Construction of a 5 World Trade Center is planned to begin in 2024 and be finished by 2029.

Christopher O. Ward, Port Authority of New York and New Jersey Executive Director from 2008 to 2011, is a survivor of the attacks and is credited with getting the construction of the 9/11 site back on track.

Memorials
In the days immediately following the attacks, many memorials and vigils were held around the world, and photographs of the dead and missing were posted around Ground Zero. A witness described being unable to "get away from faces of innocent victims who were killed. Their pictures are everywhere, on phone booths, street lights, and walls of subway stations. Everything reminded me of a huge funeral, people were quiet and sad, but also very nice. Before, New York gave me a cold feeling; now people were reaching out to help each other". President Bush proclaimed Friday, September 14, 2001, as Patriot Day. One of the first memorials was the Tribute in Light, an installation of 88 searchlights at the footprints of the World Trade Center towers. In New York City, the World Trade Center Site Memorial Competition was held to design an appropriate memorial on the site. The winning design, Reflecting Absence, was selected in August 2006, and consists of a pair of reflecting pools in the footprints of the towers, surrounded by a list of the victims' names in an underground memorial space. The memorial was completed on September 11, 2011; a museum also opened on site on May 21, 2014.

The Sphere by the German sculptor Fritz Koenig is the world's largest bronze sculpture of modern times, and stood between the Twin Towers on the Austin J. Tobin Plaza of the World Trade Center in New York City from 1971 until the terrorist attacks on September 11, 2001. The sculpture, weighing more than 20 tons, was the only remaining work of art to be recovered largely intact from the ruins of the collapsed Twin Towers after the attacks. Since then, the work of art, known in the U.S. as The Sphere, has been transformed into an important symbolic monument of 9/11 commemoration. After being dismantled and stored near a hangar at John F. Kennedy International Airport, the sculpture was the subject of the 2001 documentary The Sphere by filmmaker Percy Adlon. On August 16, 2017, the work was reinstated, installed at the Liberty Park, close to the new World Trade Center aerial and the 9/11 Memorial.

In Arlington County, the Pentagon Memorial was completed and opened to the public on the seventh anniversary of the attacks in 2008. It consists of a landscaped park with 184 benches facing the Pentagon. When the Pentagon was repaired in 2001–2002, a private chapel and indoor memorial were included, located at the spot where Flight 77 crashed into the building.

In Shanksville, a concrete-and-glass visitor center was opened on September 10, 2015, situated on a hill overlooking the crash site and the white marble Wall of Names. An observation platform at the visitor centre and the white marble wall are both aligned beneath the path of Flight 93. A temporary memorial is located 500 yd from the crash site. New York City firefighters donated a cross made of steel from the World Trade Center and mounted on top of a platform shaped like the Pentagon. It was installed outside the firehouse on August 25, 2008. Many other permanent memorials are elsewhere. Scholarships and charities have been established by the victims' families and by many other organizations and private figures.

On every anniversary in New York City, the names of the victims who died there are read out against a background of sombre music. The President of the United States attends a memorial service at the Pentagon, and asks Americans to observe Patriot Day with a moment of silence. Smaller services are held in Shanksville, Pennsylvania, which are usually attended by the First Lady. In September 2023, President Joe Biden did not attend services in the affected areas, instead marking the day in Anchorage, Alaska, the first U.S. President to do so since the attacks.