On September 11, 2001, 19 militants associated with the Islamist extremist group al Qaeda hijacked four airliners and carried out suicide attacks against targets in the United States. Two of the planes were flown into the Twin Towers of the World Trade Center in New York City, a third plane hit the Pentagon just outside Washington, D.C., and the fourth plane was crashed into a field near Shanksville Pennsylvania.

Often referred to simply as 9/11, the attacks resulted in extensive
death and destruction, triggering major U.S. initiatives to combat terrorism. Nearly 3,000 people were killed during the attacks, including more than 400 police officers and firefighters and 246 passengers and crew on the four planes. This was the most deadly terrorist attack in U.S. history.