By Mike Rudin BBC, Conspiracy Files
The final mystery of 9/11 will soon be solved, according to US experts investigating the collapse of the third tower at the World Trade Center.
Investigators are expected to say ordinary fires on several different floors caused the collapse.
However, a group of architects, engineers and scientists say the official explanation that fires caused the collapse is impossible. Architects and Engineers for 9/11 Truth argue there must have been a controlled demolition.
The founder of the group, Richard Gage, says the collapse of the third tower is an obvious example of a controlled demolition using explosives.
"Building Seven is the smoking gun of 9/11. A sixth grader can look at this building falling at virtually freefall speed, symmetrically and smoothly, and see that it is not a natural process.
There are a number of facts that have encouraged conspiracy theories about Tower Seven.
The third tower was occupied by the Secret Service, the CIA, the Department of Defence and the Office of Emergency Management, which would co-ordinate any response to a disaster or a terrorist attack.
"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."
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.
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.
BBC - Q&A: The collapse of Tower 7
The official version, the conspiracy theories and the evidence surrounding the collapse of World Trade Centre 7 on September 11.