A U.S. Navy MH-60S Sea Hawk helicopter flies by during a 9/11 remembrance ceremony at the Key West-Florida Keys Historic Military Memorial in Key West, Fla., 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.