The NIST Final Report on the Collapse of World Trade Center Building 7 states:
The fires in WTC 7 were ignited as a result of the impact of debris from the collapse of WTC 1, which was approximately 110 meters (350 feet) to the south. … The fires were ignited on at least 10 floors; however, only the fires on Floors 7 through 9 and 11 through 13 grew and lasted until the time of the building collapse. … Had a water supply for the automatic sprinkler system been available and had the sprinkler system operated as designed, it is likely that fires in WTC 7 would have been controlled and the collapse prevented. (p. xxxvi)
… both the primary and back-up source of water for the sprinkler system in the lower 20 floors of WTC 7 was the city water main. Since the collapses of the WTC towers had damaged the water main, there was no secondary supply of water available … to control those fires that eventually led to the building collapse. (p. xxxvii)
… As the fires progressed, some of the structural steel began to heat. … The heat from these uncontrolled fires caused thermal expansion of the steel beams on the lower floors of the east side of WTC 7 … damaging the floor framing on multiple floors.
The initiating local failure that began the probable WTC 7 collapse sequence was the buckling of Column 79. … When steel (or any other metal) is heated, it expands. If thermal expansion in steel beams is resisted by columns or other steel members, forces develop in the structural members that can result in buckling of beams or failures of connections. (p. 21)
Fire-induced thermal expansion of the floor system surrounding Column 79 led to the collapse of Floor 13, which triggered a cascade of floor failures … down to the 5th floor (which … was much thicker and stronger). … This left Column 79 with insufficient lateral support, and as a consequence, the column buckled eastward, becoming the initial local failure for collapse initiation. (p. 22)