On September 11, 2001, extremist groups
hijacked four airplanes 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 crashed in a field in Pennsylvania.
Almost 3,000 people were killed
during the 9/11 terrorist attacks.
These attacks triggered major U.S. initiatives
to combat terrorism.