"One thing that confounded engineers was how falling parts of the structure ploughed through undamaged building beneath and brought the towers down so quickly," said Dr Seffen.

Conspiracy theorists see evidence of a "controlled detonation"
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 behaviour of the buildings.

From Keith's link.