U.S. Air Force honor guard members assigned to the 455th Air Expeditionary Wing walk in formation during a retreat ceremony at Bagram Airfield, Afghanistan, Sept. 11, 2014, as part of an event to mark the anniversary of 9/11. Terrorists hijacked four passenger aircraft Sept. 11, 2001. Two of the aircraft were deliberately crashed into the World Trade Center in New York; one was crashed into the Pentagon; the fourth crashed near Shanksville, Pa. Nearly 3,000 people died in the attacks.