I've watched the videos many times, and it's obvious in the case of the two main towers that the top portions collapsed onto the crash points first. The rest falls from the top down. There are puffs of smoke lower down, probably from pressure as the higher portions force the air out of the floors below. It doesn't look like there are any interior explosions to me.
I don't know much about building 7, but these guys do:
http://www.popularmechanics.com/tech...42.html?page=5
Also see: