U.S. Navy Air Traffic Controller 1st Class Vleet Roberts, center, sings "God Bless America" at the conclusion of 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.