Explosive is the word they use a lot here, but the majority of them usually state that the thermite was used to "cut" strategic support beams. Either way it makes no sense. Let me hit a build with a plane loaded with "explosives", building doesn't go down at that moment, but instead seems to wait almost an hour. Thermite can be effective in controlled situations. Does a building, hit by a large passenger plane, and on fire, sound like a controlled situation? The answer is obvious.

I would suspect that alpha's post nails it on the head. You could probably dig up old used nails and other metals and detect "thermite".