NIST conducted an extremely thorough three-year investigation into what caused the WTC towers to collapse, as explained in NISTs dedicated Web site,
NIST and the World Trade Center. This included consideration of a number of hypotheses for the collapses of the towers.
Some 200 technical expertsincluding about 85 career NIST experts and 125 leading experts from the private sector and academiareviewed tens of thousands of documents, interviewed more than 1,000 people, reviewed 7,000 segments of video footage and 7,000 photographs, analyzed 236 pieces of steel from the wreckage, performed laboratory tests and sophisticated computer simulations of the sequence of events that occurred from the moment the aircraft struck the towers until they began to collapse.
Based on this comprehensive investigation, NIST concluded that the WTC towers collapsed because: (1) the impact of the planes severed and damaged support columns, dislodged fireproofing insulation coating the steel floor trusses and steel columns, and widely dispersed jet fuel over multiple floors; and (2) the subsequent unusually large jet-fuel ignited multi-floor fires (which reached temperatures as high as 1,000 degrees Celsius) significantly weakened the floors and columns with dislodged fireproofing to the point where floors sagged and pulled inward on the perimeter columns. This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers. Both photographic and video evidenceas well as accounts from the New York Police Department aviation unit during a half-hour period prior to collapsesupport this sequence for each tower.