i don't know, and i don't claim to know. but if everyone else here thinks it was debris from the other two towers, then let me ask them a question:
if debris did hit WTC7, in sufficient quantity to cause WTC7's collapse, then why did WTC7 not collapse in teh direction where this supposed debris hit? would that not be logical, that the building would collapse towards a wall that was damaged by debris, would it not collapse towards the weakest part of the building?
and yet it falls in on itself, in perfect symmetry.
if there is a logical explanation for that, then let's have it, please.