A U.S. flag waves while flying at half-staff following a 9/11 commemoration ceremony Sept. 11, 2014, at Camp Leatherneck, Afghanistan. U.S. Marines, Sailors and coalition partners attended the ceremony to commemorate the 13th anniversary of the terrorist attacks. 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.