Engineers Uncover New Details Of WTC Collapse

[E]ngineers think they are closing in on the specific structural failures that, like a few loose stones on a mountain, set off the deadly sequence of events inside the burning, partly smashed buildings. It could all come down to the sagging steel underpinnings of a few floors, which then tore away connections held by pairs of three-quarter- inch bolts and a pattern of welds where the floors were bound to columns."