WTC 7 collapsed at about 5:20 pm on the 11th. There were diesel tanks located within the building and small spot fires were seen in the building, possibly a result of debris from the South Tower.
"Building 7's tenants included the Secret Service, the Internal Revenue Service, and several financial institutions, including the Securities and Exchange Commission. The SEC lost numerous case files in the collapse -- files relating to investigations of corporations such as World Com.
Building 7 also housed Mayor Giuliani's Emergency Command Center, a bunker on the 23rd floor with blast-resistant windows and its own air and water supply. The mayor was operating out of temporary quarters at 75 Barkley Street when the towers collapsed. Supposedly, the command center was evacuated before the first tower collapsed, even though no-one -- certainly not the firemen -- had expected any of the buildings to collapse"