That theory has generally held up. A federal investigation was inconclusive, but it did conform with engineers' suspicious. The steel columns didn't actually melt. But as the fires inside the buildings consumed more and more jet fuel - about 24,000 gallons of it - they overwhelmed the buildings' sprinkle systems, started to weaken the concrete insulation around the steel, and then deliberated the steel itself. Most of the fluffy fireproofing material on the towers' steel columns was blasted off by the plane crashes. With flames reaching temperatures of 1,500 degrees Fahrenheit and higher, the steel columns started up to warp, and the top floors buckled. From then on, the towers has no more chance of standing than a building targeted for demolition. Their collapse was the first known instance of steel buildings collapsing because of fire alone.