The September 11, 2001, attacks

Terrorist attacks upon the United States of America on the morning of September 11, 2001. Nineteen members of the Al Qaeda terrorist network hijacked four commercial airliners, crashing two into the twin towers of the World Trade Center in New York City and one into the Pentagon in Virginia. The fourth plane crashed in rural Pennsylvania; its target may have been the U.S. Capitol or the White House. It was the worst terrorist attack in the history of the United States.

http://en.wikipedia.org/wiki/Al-Qaeda