Sep. 11, 2001, was a Tuesday. On that day, which is often referred to as 9/11, four planes were hijacked by terrorists to carry out suicide attacks against U.S. targets.
On Tuesday Sep. 11, 2001, two of four hijacked planes struck the World Trade Center in New York City. The third plane was flown into the Pentagon near Washington, D.C. Another plane crashed in a field in Pennsylvania after passengers attacked the hijackers who took over the airliner. More than 3,000 people were killed in the attacks at the Pentagon and World Trade Center, including more than 400 firefighters and police officers.