[tbontob]

Until recently, the most current hypothesis was that the insulation on the floor girders burned off allowing them to soften and thereby loose their ridgidity with the result that the buildings lost their integrity and ridgidity.

About a month or so ago, a TV program (I think it may have been on Discovery) said the above has fallen out of favour and the real reason was that the core around which the elevators were housed weakened to such an extent that it collapsed, which in turn resulted in the collapse of the buildings.