The September 11 attacks (also referred to as September 11, September 11th, or 9/11) were a series of four terrorist attacks launched by the Islamic terrorist group al-Qaeda against the United States in New York City and the Washington, D.C. on Tuesday, September 11, 2001. The attacks killed 2,996 people and caused at least $10 billion in property and infrastructure damage.

Four passenger airliners were hijacked by 19 al-Qaeda terrorists so they could be flown into buildings in suicide attacks. Two of those planes, American Airlines Flight 11 and United Airlines Flight 175, were crashed into the North and South towers, respectively, of the World Trade Center complex in New York City. Within two hours, both of the 110-story towers collapsed with debris and the resulting fires causing partial or complete collapse of all other buildings in the World Trade Center complex, including the 47-story 7 World Trade Center tower, as well as significant damage to ten other large surrounding buildings. A third plane, American Airlines Flight 77, was crashed into the Pentagon (the headquarters of the United States Department of Defense), leading to a partial collapse in its western side. The fourth plane, United Airlines Flight 93, was targeted at Washington, D.C. but crashed into a field near Shanksville, Pennsylvania, after its passengers tried to overcome the hijackers. In total, 2,996 people died in the attacks, including the 227 civilians and 19 hijackers aboard the four planes. It was also was the deadliest incident for firefighters and for police officers in the history of the United States, with 343 firefighters and 74 police officers killed.