Following a three-year investigation, the National Institute of Standards and Technology has determined that World Trade Center 7 was felled by fire on 9/11, hopefully putting to rest speculation about the nature of its collapse. In a detailed 115-page report [PDF] released today by the institute, a team of investigators determined that flaming debris from the collapse of Tower 1 set the blaze, not explosives or a fuel fire from back-up generators in the building.

“We knew from the beginning of our study that understanding what happened to Building 7 on 9/11 would be difficult,” Shyam Sunder, the lead investigator, said in prepared remarks. “It did not fit any textbook description that you could readily point to and say, yes, that’s why the building failed.”

The investigation team created an incredibly detailed model of the building, then set fire to it, based on visual and technical data, and watched the results. (Here are some nifty videos of said models.) The report said the critical issue was that the collapsing tower also severed the city water main, cutting service to the sprinkler system, which allowed the fire to burn for seven hours, weakening the structural steel on the World Trade Center side enough to cause the building to fall.

Explosions were ruled out because the sound waves would have been detected even a mile away. The team did acknowledge that what made the questions so compelling was the fact that no building taller than 15 stories had previously been known to collapse from fire. The report concludes that fire safety standards should be reviewed nationwide and strengthened where necessary to prevent similar collapses in the future.