President Barack Obama, center, delivers remarks as Secretary of Defense Chuck Hagel, left, and U.S. Army Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, stand by during a ceremony Sept. 11, 2014, at the Pentagon Memorial in Arlington, Va., to mark the 13th 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.