The fourth plane, United Airlines Flight 93, was initially flown toward Washington, D.C., but crashed into a field in Stonycreek Township near Shanksville, Pennsylvania, after its passengers thwarted the hijackers. 9/11 is the single deadliest terrorist attack in human history and the single deadliest incident for firefighters and law enforcement officers in the history of the United States, with 343 and 72 killed, respectively.