The September 11, 2001 attacks (often referred to as 9/11) were a series of coordinated suicide attacks by al-Qaeda upon the United States.

On that morning, nineteen terrorists affiliated with al-Qaeda hijacked four commercial passenger jet airliners. Each team of hijackers included a member who had undergone some pilot training. The hijackers intentionally crashed two of the airliners (American Airlines Flight 11 and United Airlines Flight 175) into the World Trade Center in New York City, one plane into each tower (1 WTC and 2 WTC), resulting in the collapse of both buildings soon afterward and extensive damage to nearby buildings. The hijackers crashed a third airliner (American Airlines Flight 77) into the Pentagon in Arlington County, Virginia, near Washington, D.C. Passengers and members of the flight crew on the fourth aircraft (United Airlines Flight 93) attempted to retake control of their plane from the hijackers; that plane crashed into a field near the town of Shanksville in rural Somerset County, Pennsylvania.

Aside from the 19 hijackers, 2,974 people died as an immediate result of the attacks, and the death of at least one person from lung disease was ruled by a medical examiner to be a result of exposure to WTC dust. Another 24 people are missing and presumed dead, bringing the total number of victims to 2,998, the overwhelming majority of whom were civilians. The dead included nationals from over 80 different countries.