I agree with you there. It could have been a combination of DEW, mini nukes, nano-thermite, and traditional explosives. They left nothing to chance because they wanted those buildings down at all costs.

Leaving half a building would have been a terrible outcome for the perps, the evidence would have been everywhere. A total collapse meant they could scoop up the evidence and ship it off (to China) before anybody could notice.

Those towers were not flimsy, they were tanks and probably the strongest buildings in the world. Nothing short of throwing everything they had at it would bring them down normally.