The Official Account
NIST originally suggested  that WTC 7 was brought down by structural damage combined with a raging fire fed by diesel fuel. However, in its Final Report (of November 2008),  NIST declared that neither diesel fuel nor structural damage played a role in this building’s collapse, and that this building, which was not struck by a plane, was brought down by fire alone.
The Best Evidence
Before or after 9/11, no steel-frame high-rise building had ever collapsed due to fire.  If fire were to cause such a building to collapse, the onset would be gradual, whereas the videos show that WTC 7, after being completely stable, suddenly came down in virtual free fall. This building’s straight-down, symmetrical collapse, with the roofline remaining essentially horizontal, shows that all 82 of WTC 7’s support columns had been eliminated by the time the top started down.
References for WTC 7 Point 1
- FEMA, World Trade Center Building Performance Study, Ch. 5, Sect. 6.2, “Probable Collapse Sequence.”
Debunking 9/11 Myths: Why Conspiracy Theories Can’t Stand Up to the Facts: An In-Depth Investigation by “Popular Mechanics”, ed. David Dunbar and Brad Reagan (New York: Hearst Books, 2006), 53-58. This semi-official book contained quotations from NIST showing what it was thinking at the time.
- NIST NCSTAR 1A, Final Report on the Collapse of World Trade Center Building 7 (archive copy), November 2008, 49 (pdf: 91). In NIST’s words, the collapse of WTC 7 was “the first known instance of the total collapse of a tall building primarily due to fires.”
- See a discussion of other skyscraper fires, and see videos here, here, and here.