"Would it be crazy to suggest that al Qaeda had a back-up plan to make sure that the towers fell?"
I often wondered about this. I've read a lot of the "conspiracy" sites and such.
How hard would it have been for the terrorists to a) plant bombs before the strike or b) dress as first responders and plant bombs during the confusion -- it's not like they would be ones worried about getting out. With everyone focused on rescue I think it may have been rather easy with the right "props" to get in there...
Planting the bombs to make sure they fell would also be "good" from the terrorists point of view as they would not only get the civilians, but first responders as well thereby causing more panic, confusion, and pain. It's not like they didn't know what to do in regards to planting bombs in the lower levels of the WTC, as they did it before.
[NOTE: I hate that any divergent thinking is automatically labeled a conspiracy theory. Granted, some are pretty dang far-fetched, but others leave one with serious questions and I don't think it does anyone any good to dismiss some of the research that's been done as pure BS just because it's not EXACTLY what the authorities think happened.]