NEW YORK (AFP) - A raging, long-burning fire caused the collapse of World Trade Centre building number 7, which tumbled hours after hijacked jets hit the twin towers on September 11, 2001, investigators said Thursday. The National Institute of Standards and Technology (NIST) found that the 47-storey building, which stood right next to towers 1 and 2, caught fire due to the debris that flew into it after the planes struck. It eventually fell after seven hours due to instability caused by the fire, not by explosives as some conspiracy theorists have claimed. "Our study found that the fires in WTC 7, which were uncontrolled but otherwise similar to fires experienced in other tall buildings, caused an extraordinary event," said lead investigator Shyam Sunder. "Heating of floor beams and girders caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down," he said. "Video and photographic evidence combined with detailed computer simulations show that neither explosives nor fuel oil fires played a role in the collapse of WTC 7." He said debris from tower 1 sparked the blaze on at least 10 floors of the  building, which burned for about seven hours while some of its sprinkler systems were non-operational due to a city water main cut. "We did not find any evidence that explosives were used to bring the building down," he said. "This size blast would have produced an incredibly loud sound that was not recorded on videos of the collapse nor reported by witnesses." He added that the "collapse was also not due to fires from the substantial amount of diesel fuel stored in the building." A team of 50 experts from diverse fields, including engineering, explosives, fire, and construction took part in the probe. The September 11 attacks on New York killed nearly 3,000 people.