September 11, 2001:

19 al Qaeda members hijacked four US passenger airliners. One would hit the Pentagon in Washington, D.C., another would crash into the Pennsylvania countryside after passengers tried to take control of the plane headed towards Washington, D.C., and two would crash into the Twin Towers in New York. They are considered the most lethal terrorist attacks in history and would greatly alter anti-terror approaches around the world, as the U.S. launched the War on Terror. —FBI

Impact:

• Nearly 3,000 fatalities
• More than 6,000 injured

Related Resources:

• HSDL In Focus: September 11, 2001 Attacks
• Public Law 107-39: Joint Resolution Expressing the Sense of the Senate and House of Representatives Regarding the Terrorist Attacks Launched against the United States on September 11, 2001
• To the Congress of the United States [September 10, 2009]
• Attacks by Al Qaeda both Before and After the September 11, 2001 Attack on the US
• HSDL search results for: September 11, 2001 Attacks

Photo: (Credit: Robert J. Fisch/Flickr User: TheMachineStops) North face, south tower after plane strike.