Secretary of Defense Chuck Hagel, right, gives remarks prior to introducing President Barack Obama during a memorial ceremony in honor of the 13th anniversary of the attack on the Pentagon Sept. 11, 2014, at the Pentagon Memorial in Arlington, Va. 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.