The specifics of the fires in WTC 7 and how they caused the building to collapse remain unknown at this time. Although the total diesel fuel on the premises contained massive potential energy, the best hypothesis has only a low probability of occurrence. Further research, investigation, and analyses are needed to resolve this issue."[/I] FEMA
source
Dr S. Shyam Sunder, NIST's lead WTC disaster investigator, said, of 7 World Trade Center, "We are studying the horizontal movement east to west, internal to the structure, on the fifth to seventh floors”; he added "But truthfully, I don’t really know. We’ve had trouble getting a handle on Building No. 7".
wikipedia