A collision perhaps... but no material that we use in building today is gonna withstand how many hundreds of gallons of burning, exploding plane fuel gushing though the ventilation ducts, the stairwells, and the elevator shafts... it was that fire that melted steel and concrete and glass and caused the buildings to collapse the way they did...

Something, I suspect, that even the terrorists didn't expect to happen... Which could explain why no one is quick to claim responsibility... I donno... I'm just thinking out loud...