From left, President Barack Obama, Secretary of Defense Chuck Hagel and Chairman of the Joint Chiefs of Staff U.S. Army Gen. Martin E. Dempsey participate in a 9/11 commemoration ceremony at the National 9/11 Pentagon Memorial in Arlington, Va., Sept. 11, 2014. 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.