U.S. Air Force Heartland of America Band, Air Combat Command, performs before the 9/11 memorial service held in SAC Chapel on Offutt Air Force Base, Neb., Sept. 11. On Sept. 11, 2001, 19 militants associated with the Islamic extremist group al-Qaeda hijacked four airliners and carried out suicide attacks against targets in the United States. Two of the planes were flown into the towers of the World Trade Center in New York City, a third plane hit the Pentagon just outside Washington, D.C., and the fourth plane crashed in a field in Pennsylvania. (U.S. Air Force photo by Charles Haymond/Released)