FEMA couldn ' t come to a conclusion as to the cause other than fire . The NIST investigation , which wasn ' t completed until 7 years later says NIST determined that diesel fuel did not play an important role , nor did the structural damage from the collapse of the Twin Towers or the transfer elements trusses , girders , and cantilever overhangs . The lack of water to fight the fire was an important factor . The fires burned out of control during the afternoon , causing floor beams near column 79 to expand and push a key girder off its seat , triggering the floors to fail around column 79 on Floors 8 to 14 . With a loss of lateral support across nine floors , column 79 buckled pulling the east penthouse and nearby columns down with it . With the buckling of these critical columns , the collapse then progressed easttowest across the core , ultimately overloading the perimeter support , which buckled between Floors 7 and 17 , causing the remaining portion of the building above to fall downward as a single unit . The fires , which were fueled by office contents and burned for 7 hours , along with the lack of water , were the key reasons for the collapse .