 The fatal blow to the building came when the 13th floor collapsed, weakening a critical steel support column that led to catastrophic failure, and extreme heat caused some steel beams to lose strength, causing further failures throughout the buildings until the entire structure succumbed.Also cited as a factor was the collapse of the nearby towers, which broke the city water main, leaving the sprinkler system in the bottom half of the building without water.NIST considered the possibility that 7 WTC was brought down with explosives and concluded that a blast event did not occur, that the "use of thermite [...] to sever columns in 7 WTC on 9/11/01 was unlikely".