Take a look here, pretty cool, let's you go through one of the collapses frame by frame:
http://www.911research.wtc7.net/wtc/...ts_frames.html
Pay particular attention to frames 3 and 7. There appears to be highly concentrated blasts of energy exiting the building several floors below the current location of the collapse. I would guess these are shooting out about 20 feet, but that's just a guess.
Anyway, the collapse is officially blamed on pancaking. If this is the case, what are these blasts of energy?