NIST’s account has four steps. The planes severed and damaged columns, knocked fireproofing off the steel and spread jet fuel over several floors. The fires that followed, fed mostly by office contents and reaching about 1,000°C, heated the unprotected steel. The floors sagged and pulled inward on the perimeter columns, which bowed in. The south face of the North Tower and the east face of the South Tower failed first. NIST says photographs, video and reports from the New York police aviation unit in the half hour before each collapse support that sequence. [1][2]
When the bowed columns buckled, the whole section of the building above the impact zone began to move down. NIST says that once that happened, the structure below could not stop it. The collapse began at the impact and fire floors and nowhere else, and the time it took to start, 56 and 102 minutes, depended on how much damage each plane did and how long the fires took to reach the critical areas. [2]
About 200 experts worked on the investigation. They reviewed tens of thousands of documents, interviewed more than 1,000 people, studied 7,000 segments of video and 7,000 photographs, and analyzed 236 pieces of recovered steel. [2]
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