One of the main reasons for the speed of the Twin Towers collapse is that the New York planning authorities, for reasons of cost and expediency, permitted the original construction to go ahead with insufficient fire escape routes. In England, a high-rise building must have concrete-encased stairwells that resist fire for, I think, two hours and which provide safe, rigid escape routes. Structurally, they often form the trunk of the building, from which lighter steel/concrete structures hang out like branches; that`s how you can have walls of glass, for instance.
In the Twin Towers, the stairwells weren`t concrete-encased, they were just braced with steel beams with some kind of fire-resistant covering, which didn`t prevent the steel getting hot. Unfortunately, when steel gets hot it loses strength even faster than burning timber. Look for the 20-minutes-into-the-fire line on the bottom axis of this graph:-
After 20 mins, a piece of timber still has 50% of its strength; a piece of steel has about 7% - it`s about as strong as Jell-O. That`s why the buildings appeared to implode; they just collapsed under their own weight.
Of course, after the event, no-one was very keen to put up their hand and say, "I allowed that building to be constructed on the cheap", so in the US there might`ve been something of a hushing-up conspiracy. In Britain, though, all of the above was reported in a very reputable construction-trade journal about one year after the tragic events of 9/11. ( May all those poor people R.I.P.)
^ I so hope neither of these are true.