I feel pretty sure about this, that the planes were just a diversion set to allow emergency vehicles easy access to the basement parking lot. Eye witnesses placed a large explosion just before the south building came down.
One thing is sure, the buildings did not come down as a result of the planes hitting them or any explosions due to that. Even the attempt to bring down a tower a few years ago proved unsuccessful and there was one hell of a lot more power in that blast. The action necessary to bring down the towers had to be at the base and had to be planned just right.
The terrorists would never have been able to get that much explosives into place under normal circumstances.
I believe that fake ambulances showed up prior to the real ones and were waved on through by security, whose guard was down due to circumstance.