It wasn't explosives and it wasn't burning jet fuel.
The fall of the 47-story World Trade Center's Building 7 at 5:20 p.m. on Sept. 11, 2001, was primarily due to fires from the building itself, NIST announced Friday, following an extensive, three-year investigation. This was the first known instance of fire causing the total collapse of a tall building, the agency said.
"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."
The trapezoid-shaped Building 7 was connected to the World Trade Center plaza by an elevated walkway. The building was damaged and set afire by debris - including an enormous broadcast antennae - from the collapse of the nearby North Tower; it burned all day and collapsed itself late that afternoon.
"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," Mr. Sunder said.
The NIST investigation team also determined that other elements of the building´s construction - namely trusses, girders and cantilever overhangs - did not play a significant role in the collapse.
Another key factor was "thermal expansion" of the structural floor systems - and lack of water to fight the fires.
The agency issued a set of 13 recommendations to the nation's builders - many echoed in previous investigations of the destruction of the other World Trade Center structures. Floors, framing and structural connections were particularly at issue, the report said.
Construction on a new "7 World Trade Center" began in 2002 and was completed four years later. The building is 52 stories tall, though with a smaller footprint than the original.
The new design, billed as the nation's "safest skyscraper" by owner Silverstein Properties, features extensive fireproofing measures, a two-foot thick reinforced concrete core and wider stairways- four of which exit directly outside the building. Interior stairwells feature emergency lighting, glow-in-the-dark paint and are pressurized to purge smoke in an emergency.
"7" boasts a certification from U.S. Green Building Council's Leadership in Energy and Environmental Design program as Manhattan's first "green office tower." Thirty percent of the structural steel and much of the interior insulation is recycled steel, while collected rainwater is used to irrigate a nearby park and help cool the building.