U.S. flags flew at half-mast at the Pentagon on Sept. 12, 2001, while firefighters continued to extinguish flames after a highjacked commercial jetliner crashed into the building the day before. The terrorist attack caused extensive damage to the Pentagon and followed similar attacks on the twin towers of the World Trade Center in New York City. (U.S. Air Force file photo/Master Sgt. Jim Varhegyi)