A fire that broke after the terrorist attacks on the World Trade Center on September 11, 2001 had caused the collapse of a third skyscraper, located about 150 meters north of the Twin Towers (WTC 1 and 2).

The details about the fall of the 47-story World Trade Center building 7 (WTC 7) in the late afternoon of Sept. 11, 2001 and its causes were announced Thursday in a report prepared by the US Commerce Department's National Institute of Standards and Technology (NIST) following an extensive, three-year scientific and technical building and fire safety investigation.

The collapse of WTC 7 is the first known instance of fire causing the total collapse of a tall building, the agency stated as it released for public comment its WTC investigation report and 12 recommendations for improving building and fire safety.

There have been no casualties due to the collapse of WTC 7 because its 4,000 employees have been evacuated on time. The building included the offices of the CIA and the Secret Services in New York as well as the Center for Emergency Situations of the New York City Hall.

NIST has found that the impact of debris from the collapse of WTC 1 ignited fires on at least 10 floors of WTC 7, and the fires burned out of control on six lower floors. The heat from these uncontrolled fires caused thermal expansion of the steel beams on the lower floors, damaging the floor framing on multiple floors. Eventually, a girder on Floor 13 lost its connection to a critical interior column that provided support for the long floor spans. The displaced girder and other local fire-induced damage caused Floor 13 to collapse, beginning a cascade of floor failures down to the fifth floor. This collapse of floors left the critical column unsupported over nine stories.

The initial hypothesis for the possibility of other factors playing a role in the collapse of WTC 7, including the possible use of explosives, fires fed by the fuel supply tanks in and under the building, and damage from the falling debris of WTC 1 have not been confirmed by the NIST investigation.

The fact that the collapse of a third tower remained unmentioned for quite a while and the earlier versions about explosives in the building have stirred quite a few "conspiracy theories" regarding the collapse of Tower 7 in particular and the September 11 events in general.