User:Cs32en/911/Overview/Sources

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People at the scene, print journalists,  policemen, firefighters, and television news anchors Peter Jennings and Dan Rather have compared the process of the destruction of the World Trade Center buildings to that of a controlled demolition, or have reported explosions.

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Some of the steel from the Twin Towers was removed and sent to scrap yards before engineers were allowed access to the site on October 6, 2001. Webster Griffin Tarpley, an author, has criticized the official response to the crime scene, saying that the cleanup process resulted in the destruction of most of the evidence, identifying the New York City Mayor's office as a key player in this regard.

The debris removal process began shortly after the attacks, and concluded in May 2002. Robert F. Shea of FEMA testified to the House of Representatives that, "Because of the importance of the rescue effort at the World Trade Center complex, it was clear that information would have to be gathered without interfering with response and rescue activities. Based on this fact, the FEMA-ASCE team first visited the site on October 6, [2001] but gathered information from others who had been on-site before this date."

A call to action by Bill Manning, the chief editor of the trade journal Fire Engineering, is often quoted in this connection. In a January 2002 editorial, Manning called the early ASCE investigation (which would later turn into the FEMA building performance study) a "half-baked farce" and said that "the destruction and removal of evidence must stop immediately." He said that the cleanup of the WTC site differed in many respects from that of other engineering disasters. In defense of the decision to dispose of the steel, Mayor Bloomberg said: "If you want to take a look at the construction methods and the design, that's in this day and age what computers do." David Ray Griffin notes that this is exactly what Manning had worried about when he warned that "the investigation into the World Trade Center fire and collapse will amount to paper-and computer-generated hypotheticals.

However, allegations against a "speedy removal" of the steel hampering the engineering investigations appear to be unfounded, according to Dr. Gene Corley, head of the BPAT team and one of the lead engineers for the investigation by the National Institute of Standards and Technology (NIST), which began in September 2002. He testified to the House of Representatives in March 2002 that, "There has been some concern expressed by others that the work of the team has been hampered because debris was removed from the site and has subsequently been processed for recycling. This is not the case. The team has had full access to the scrap yards and to the site and has been able to obtain numerous samples. At this point there is no indication that having access to each piece of steel from the World Trade Center would make a significant difference to understanding the performance of the structures".

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The controlled demolition theory is also offered to explain a belief that the towers collapsed close to free fall speed. Most estimates agree that the structures offered little resistance to the progress of the collapses and that they took about 50% longer than a free falling object dropped from the tops of the towers. Without explosives to destroy the internal support structure of the WTC towers, argue proponents of controlled demolition, the fall of the towers would violate the principle of conservation of momentum. Others say that these claims are only supported by intuition without any quantitative analysis. They point to their own analyses posted on a website suggesting that the fall may be explained without violating the principle of conservation of momentum and without requiring any explosives.