KEESLER AIR FORCE BASE, Miss. --   Staff Sgt. Anthony Marshall, 81st Infrastructure Division firefighter, conducts a last alarm tribute during a 9/11 memorial retreat ceremony, Sept. 12, 2016, on Keesler Air Force Base, Miss. Fifteen years ago on Sept. 11, 2001, 19 terrorists simultaneously hijacked four passenger jets; flying one into the Pentagon and two into the World Trade Center in New York City, N.Y., and one crashing outside of Shanksville, Pa. A total of 2,996 people were killed in the attacks. (U.S. Air Force photo by Kemberly Groue/Released)