Update:
Structure Magazine, a well respected magazine for structural engineers, has come out with a probable collapse hypothesis. "Single Point of Failure: How the Loss of One Column May Have Led to the Collapse of WTC 7" points out that the failure of column 79 in the lower levels will create the very effect we see in videos.
http://www.structuremag.org/Archives/2007-11/SF-WTC7-Gilsanz-Nov07.pdf
Yet another peer reviewed paper from a respected Journal finds the towers were doomed to collapse.
9/11 demolition theory challenged
An analysis of the World Trade Center collapse has challenged a conspiracy theory surrounding the 9/11 attacks.
The study by a Cambridge University, UK, engineer demonstrates that once the collapse of the twin towers began, it was destined to be rapid and total.
One of many conspiracy theories proposes that the buildings came down in a manner consistent with a "controlled demolition".
The new data shows this is not needed to explain the way the towers fell.
Resistance to collapse
Dr Keith Seffen set out to test mathematically whether this chain reaction really could explain what happened in Lower Manhattan six years ago. The findings are published in the Journal of Engineering Mechanics.
Previous studies have tended to focus on the initial stages of collapse, showing that there was an initial, localized failure around the aircraft impact zones, and that this probably led to the progressive collapse of both structures.
In other words, the damaged parts of the tower were bound to fall down, but it was not clear why the undamaged building should have offered little resistance to these falling parts.
"The initiation part has been quantified by many people; but no one had put numbers on the progressive collapse," Dr Seffen told the BBC News website.
Dr Seffen was able to calculate the "residual capacity" of the undamaged building: that is, simply speaking, the ability of the undamaged structure to resist or comply with collapse.
His calculations suggest the residual capacity of the north and south towers was limited, and that once the collapse was set in motion, it would take only nine seconds for the building to go down.
This is just a little longer than a free-falling coin, dropped from the top of either tower, would take to reach the ground.
He added that his calculations showed this was a "very ordinary thing to happen" and that no other intervention, such as explosive charges laid inside the building, was needed to explain the behavior of the buildings.
The controlled detonation idea, espoused on several internet websites, asserts that the manner of collapse is consistent with synchronized rows of explosives going off inside the World Trade Center.
This would have generated a demolition wave that explained the speed, uniformity and similarity between the collapses of both towers.
Conspiracy theorists assert that these explosive "squibs" can actually be seen going off in photos and video footage of the collapse. These appear as ejections of gas and debris from the sides of the building, well below the descending rubble.
Other observers say this could be explained by debris falling down lift shafts and impacting on lower floors during the collapse.