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According to the World Trade Center controlled demolition hypothesis, the collapse of the World Trade Center on September 11, 2001, did not result from the impacts of the planes that crashed into the buildings or from the fires that resulted from them, but from explosives or incendiaries that were present in the buildings in advance. The hypothesis is a central element of alternative theories about the September 11 attacks. It is rejected by the National Institute of Standards and Technology, which was tasked with the official investigation into the collapses, as well as by specialists in structural mechanics and structural engineering, who generally accept the model of a fire-induced, gravity-driven collapse of the World Trade Center buildings, an explanation that does not involve the use of explosives.

Proponents of the hypothesis, such as physicist Steven E. Jones, architect Richard Gage and theologian David Ray Griffin, state that the aircraft impacts and resulting fires could not have weakened the buildings sufficiently to initiate a catastrophic collapse, and that the buildings would not have collapsed completely, nor at the speeds and in the way that they did, without additional energy involved to destroy or weaken their structures. Jones and Gage argue that thermite or super-thermite has likely been used to initiate the collapse of the buildings.

History
Controlled demolition conspiracy theories were first suggested in October 2001. Eric Hufschmid's book Painful Questions: An Analysis of the September 11th Attack, in which the controlled demolition theory is explicitly advocated, was published in September 2002. David Ray Griffin and Steven E. Jones are the two most prominent advocates of the theory. Griffin's book The New Pearl Harbor, published in 2004, has become a reference work for the 9/11 Truth movement. In the same year, Griffin published the book The 9/11 Commission Report: Omissions and Distortions, in which he argues that flaws in the commission's report amount to a cover-up by government officials and says that the Bush administration was complicit in the 9/11 attacks.

Steven E. Jones has become the leading academic voice of the proponents of the theories. In 2006, he published the paper "Why Indeed Did the WTC Buildings Completely Collapse?". Brigham Young University responded to Jones's "increasingly speculative and accusatory" statements by placing him on paid leave, and thereby stripping him of two classes, in September, 2006, pending a review of his statements and research. Six weeks later, Jones retired from the university. The structural engineering faculty at the university issued a statement which said that they "do not support the hypotheses of Professor Jones".

David Ray Griffin has questioned the "pancake collapse" theory suggested in the Building Performance Study produced by the Federal Emergency Management Agency (FEMA). NIST's report on the collapse of the WTC towers rejected the theory in favor of the column failure theory. In its final report, NIST stated that it "found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to September 11, 2001" and posted a FAQ about related issues to its website in August 2006. The major elements of the theory have been rebutted in mainstream engineering scholarship, where its proponents are considered "outsiders". The magazine Popular Mechanics challenged the theories in the special report "Debunking the 9/11 Myths".

The internet and homemade videos have contributed to the growth of the movement associated with the conspiracy theory that explosives had been planted in the three buildings of the World Trade Center, and the theory is often associated with allegation that the U.S. government had planned the destruction of the WTC in order to justify the invasion of Iraq and Afghanistan. The theory features prominently in the movie Loose Change. The two-hour movie 9/11: Blueprint for Truth, which is popular in the 9/11 Truth movement, is based on a presentation by San Francisco-area architect Richard Gage.

In 2006, the magazine New York reported that a "new generation of conspiracy theorists is at work on a secret history of New York’s most terrible day." The theory has been cited by popular actors, musicians and politicians, including Charlie Sheen, Willie Nelson, former Governor of Minnesota, Jesse Ventura, and talkshow host Rosie O'Donnell.

A 2006 poll found that 6 percent of Americans considered it "very likely" that "the collapse of the twin towers in New York was aided by explosives secretly planted in the two buildings", another 10% found it "somewhat likely", and 77% found the demolition theory "unlikely".

Main towers
On September 11, the North Tower (1 WTC) was hit by American Airlines Flight 11 and the South Tower (2 WTC) was hit by United Airlines Flight 175, both Boeing 767 aircraft. The South Tower collapsed 56 minutes after the impact, and the North Tower collapsed 102 minutes after. An investigation by NIST concluded that the collapse was caused by a combination of damage to support columns and fire insulation from the aircraft impacts and the weakening of columns and floors by jet fuel ignited fires. NIST also found "no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to September 11, 2001".

Steven Jones has claimed that anecdotal evidence of molten steel found in the rubble of the collapse and a stream of molten metal that poured out of the South Tower before it collapsed are evidence of temperatures beyond those produced by the fire (which was not expected to be hot enough to melt steel). Jones has argued that the molten metal may have been elemental iron, a product of a thermite reaction. Jones and other researchers analyzed samples of what they claim to be dust from the World Trade Center buildings and reported what they said was clear evidence of nano-thermite in the dust. As Jones informed NIST of his findings, NIST countered that there was no "clear chain of custody" proving that the dust indeed came from the WTC site. Jones invited NIST to conduct its own studies with dust under custody by NIST itself.

NIST found that the condition of the steel in the wreckage of the towers does not provide conclusive information on the condition of the building before the collapse and concluded that the material coming from the South Tower was molten aluminum from the plane, which would have melted at lower temperatures than steel. NIST also pointed out that cutting through the vertical columns would require planting an enormous amount of explosives inconspicuously in highly secured buildings, then igniting it remotely while keeping it in contact with the columns. A test performed by the Energetic Materials Research and Testing Center showed that conventional thermite was unable to melt a column much smaller than those used in the World Trade Center.

The NIST report provides an analysis of the structural response of the building only up to the point where collapse begins, and asserts that the enormous kinetic energy transferred by the falling part of the building makes progressive collapse inevitable once an initial collapse occurs. A paper by Zdeněk Bažant indicates that once collapse began, the kinetic energy imparted by a falling upper section onto the floor below was an order of magnitude greater than that which the lower section could support.

Engineers who have investigated the collapses generally disagree that controlled demolition is required to understand the structural response of the buildings. While the top of one of the towers did tilt significantly, it could not ultimately have fallen into the street, they argue, because any such tilting would place sufficient stress on the lower story (acting as a pivot) that it would collapse long before the top had sufficiently shifted its center of gravity. Indeed, they argue, there is very little difference between progressive collapse with or without explosives in terms of the resistance that the structures could provide after collapse began. Controlled demolition of a building to code requires weeks of preparation, including laying large quantities of explosive and cutting through beams, which would have rendered the building highly dangerous and which would have to be done without attracting the attention of the thousands of people who worked in the building. Controlled demolition is done from the bottom of buildings, not the top. There is little dispute that the collapse started high up at the point where the aircraft struck. Furthermore, any explosives would have to withstand the impact of the airliners.

Members of the group Scholars for 9/11 Truth have collected eyewitness accounts of flashes and loud explosions immediately before the fall. There are many causes of loud sharp noises that are not caused by explosives, and seismographic records of the collapse do not show evidence of explosions. Physicist Steven E. Jones and others have argued that horizontal puffs of smoke seen during the collapse of the towers would indicate that the towers had been brought down by controlled explosions. NIST attributes these puffs to pressure exerted by the falling mass of the building.

7 World Trade Center
7 World Trade Center was a 47-story skyscraper that stood across Vesey Street north of the main part of the World Trade Center site. Though not hit by a plane, it was hit by debris from the WTC towers and damaged by fires which burned for seven hours, until it collapsed at about 5:20 p.m. on the evening of September 11. Several videos of the event exist in the public domain thus enabling comparative analysis from different angles of perspective.

Some proponents of World Trade Center controlled demolition conspiracy theories suggest that 7 WTC was demolished because it served as an operational center for the alleged conspiracy, while others believe the government also wanted to destroy key files held there about corporate fraud. According to a statement reported by the BBC, Dylan Avery thinks the building was suspicious because it had some unusual tenants such as a CIA field office and several government agencies. The former chief counter-terrorism adviser to the President, Richard Clarke, does not think that 7 WTC is mysterious, and said that anyone could have rented floor space in the building.

No steel frame high rise had ever before collapsed because of a fire. BBC News reported the collapse of 7 WTC twenty minutes before it actually fell. The BBC has stated that many news sources were reporting the imminent collapse of 7 WTC on the day of the attacks. Jane Standley, the reporter who announced the collapse prematurely, called it a "very small and very honest mistake" caused by her thinking on her feet after being confronted with a report she had no way of checking.

Steven Jones says the debris contained sulfuric compounds, suggesting that thermate, a mixture of thermitic materials and sulfur, might have been used to destroy the building. He asserts that videos show a yellow molten substance, which he identifies as iron, splashing off the side of the South Tower about 50 minutes after the airplane's impact. According to Jones, pictures also show white ash of aluminium-oxide, another product of the thermite reaction. Professor Richard Sisson of Worcester Polytechnic Institute thinks the sulfur came from gypsum in the wallboards. The NIST's question and answer page for the 7 WTC investigation also mentions the presence of sulphur in the gypsum wallboard, and states that an analysis of the steel for thermite or thermate would therefore not necessarily be conclusive.

In the PBS documentary America Rebuilds, which aired in September 2002, Larry Silverstein, the owner of 7 WTC and leaseholder and insurance policy holder for the remainder of the WTC Complex, recalled a discussion with the fire department in which doubts about containing the fires were expressed. Silverstein recalled saying, "We've had such terrible loss of life, maybe the smartest thing to do is pull it". "They made that decision to pull", he recalled, "and we watched the building collapse." Silverstein issued a statement that it was the firefighting team, not the building, that was to be pulled.

In 2002 the National Institute of Standards and Technology (NIST) began a general investigation into the collapse of the World Trade Center but soon made a decision to focus first on the collapse of the Twin Towers. A draft version of its final report on the collapse of 7 WTC was released in August 2008. The agency has blamed the slowness of this investigation on the complexity of the computer model it used, which simulated the collapse from the moment it begins all the way to the ground; and NIST notes that the time taken on the investigation into 7 WTC is comparable to the time taken to investigate an aircraft crash. The agency also notes another 80 boxes of documents related to 7 WTC were found and had to be analyzed. These delays fueled suspicion the agency was struggling to come up with a plausible conclusion.

Following a three-year investigation NIST released its final report on the collapse on November 20, 2008. Investigators used videos, photographs and building design documents to come to their conclusions. The report concluded that the building collapsed due to the effects of the fires which burned for almost seven hours. The fatal blow to the building came when the 13th floor collapsed, weakening a critical steel support column that led to catastrophic failure, and extreme heat caused some steel beams to lose strength, causing further failures throughout the buildings until the entire structure succumbed. Also cited as a factor was the collapse of the nearby towers, which broke the city water main, leaving the sprinkler system in the bottom half of the building without water.

NIST considered the possibility that 7 WTC was brought down with explosives. It concluded that a blast event did not occur and that the "use of thermite [...] to sever columns in 7 WTC on 9/11/01 was unlikely". The investigation noted that no blast was audible on recordings of the collapse and that no blast was reported by witnesses, even though it would have been audible at a level of at least 130-140 decibels at a distance of half a mile. NIST also concluded that it is unlikely that the quantities of thermite needed could have been carried into the building undetected. The theory that fires from the large amount of diesel fuel stored in the building caused the collapse was also investigated and ruled out.

World Trade Center developer Larry Silverstein said, "Hopefully this thorough report puts to rest the various 9/11 conspiracy theories, which dishonor the men and women who lost their lives on that terrible day." Richard Gage, leader of the group Architects & Engineers for 9/11 Truth said, "How much longer do we have to endure the coverup of how Building 7 was destroyed?". James Quintiere, professor of fire protection engineering at the University of Maryland, who does not believe explosives brought down the towers, questioned how the agency came to its conclusions, remarking, "They don't have the expertise on explosives." Quintiere said NIST wasted time employing outside experts to consider it.

Reactions
Specialists in structural mechanics and structural engineering generally accept the model of a fire-induced, gravity-driven collapse of the World Trade Center buildings, an explanation that does not involve the use of explosives. Northwestern University Professor of Civil Engineering Zdeněk Bažant, who was the first to offer a published peer reviewed theory of the collapses, wrote "a few outsiders claiming a conspiracy with planted explosives" as an exception. Bažant and Verdure trace such "strange ideas" to a "mistaken impression" that safety margins in design would make the collapses impossible. One of the effects of a more detailed modeling of the progressive collapse, they say, could be to "dispel the myth of planted explosives". Indeed, Bažant and Verdure have proposed examining data from controlled demolitions in order to better model the progressive-collapse of the towers, suggesting that progressive collapse and controlled demolition are not two separate modes of failure (as the controlled demolition conspiracy theory assumes).

Thomas Eagar, a professor of materials science and engineering at the Massachusetts Institute of Technology, also dismissed the controlled demolition conspiracy theory. Eagar remarked, "These people (in the 9/11 truth movement) use the 'reverse scientific method.' They determine what happened, throw out all the data that doesn't fit their conclusion, and then hail their findings as the only possible conclusion."

Preparing a building for a controlled demolition takes considerable time and effort. The tower walls would have to be opened on dozens of floors. Thousands of pounds of explosives, fuses and ignition mechanisms would need to be snuck past security and placed in the towers without the tens of thousands of people working in the World Trade Center noticing. Referring to a conversation with Stuart Vyse, a professor of psychology, an article in the Hartford Advocate asks, "How many hundreds of people would you need to acquire the explosives, plant them in the buildings, arrange for the airplanes to crash [...] and, perhaps most implausibly of all, never breathe a single word of this conspiracy?"