But now nearly seven years after 9/11 the definitive official explanation of what happened to Tower Seven is finally about to be published in America.
The National Institute of Standards and Technology has spent more than two years investigating Tower Seven but lead investigator Dr Shyam Sunder rejects criticism that it has been slow.
The collapse of Tower 7
"We've been at this for a little over two years and doing a two or two and a half year investigation is not at all unusual. That's the same kind of time frame that takes place when we do aeroplane crash investigations, it takes a few years."
With no steel from Tower 7 to study, investigators have instead made four extremely complex computer models worked out to the finest detail. They're confident their approach can now provide the answers. Dr Sunder says the investigation is moving as fast as possible.
"It's a very complex problem. It requires a level of fidelity in the modelling and rigour in the analysis that has never been done before."
Other skyscrapers haven't fully collapsed before because of fire. But NIST argues that what happened on 9/11 was unique.
Steel structure weakened
It says Tower Seven had an unusual design, built over an electricity substation and a subway; there were many fires that burnt for hours; and crucially, fire fighters could not fight the fires in Tower 7, because they didn't have enough water and focused on saving lives.
Investigators have focused on the east side where the long floor spans were under most stress.
They think fires burnt long enough to weaken and break many of the connections that held the steel structure together.
Most susceptible were the thinner floor beams which required less fireproofing, and the connections between the beams and the columns. As they heated up the connections failed and the beams sagged and failed, investigators say.
The collapse of the first of the Twin Towers does not seem to have caused any serious damage to Tower Seven, but the second collapse of the 1,368ft (417m) North Tower threw debris at Tower Seven, just 350ft (106m) away.
Tower Seven came down at 5.21pm. Until now most of the photographs have been of the three sides of the building that did not show much obvious physical damage. Now new photos of the south side of the building, which crucially faced the North Tower, show that whole side damaged and engulfed in smoke.
