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 North Tower threw debris at Tower Seven, just 350ft 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.Photographs of the south side of the building, which crucially faced the North Tower, show that whole side damaged and engulfed in smoke.
No conspiracy.. and no casualties because it had long been evacuated.