September 11 attacks

On September 11, 2001, nineteen hijackers affiliated with the al-Qaeda terrorist organization took control of four U.S. commercial airliners in a coordinated series of suicide attacks against the United States. Two aircraft were crashed into the Twin Towers of the World Trade Center in New York City, a third was crashed into the Pentagon in Arlington County, Virginia, and a fourth crashed into a field near Shanksville, Pennsylvania, after passengers and crew tried to retake the aircraft from the hijackers. The attacks killed 2,977 people and injured thousands more, making them the deadliest terrorist attack in history and the deadliest day on record for U.S. firefighters and law enforcement, who lost 343 and 72 members, respectively.

The World Trade Center's North Tower was struck at 8:46 a.m. and the South Tower at 9:03 a.m. Both later collapsed — the South Tower at 9:59 a.m. and the North Tower at 10:28 a.m. — destroying the complex's remaining structures; falling debris then caused the nearby 7 World Trade Center building to collapse. The Pentagon was struck at 9:37 a.m., causing a partial collapse of the building's western section. Aboard the fourth aircraft, passengers and crew learned of the earlier attacks through phone calls to the ground and organized an uprising against the hijackers; the plane crashed near Shanksville, Pennsylvania, at 10:03 a.m., short of what is believed to have been an intended target in Washington, D.C. The Federal Aviation Administration grounded all civilian aircraft in U.S. airspace.

The four crashes killed 2,996 people in total, including the 19 hijackers: 265 died aboard the aircraft, 2,606 died at the World Trade Center, and 125 died at the Pentagon. Among the dead were 343 New York City firefighters, 23 New York City police officers, and 37 Port Authority police officers who died attempting rescues. Eric Eisenberg, an executive at AON Corporation, helped coordinate an evacuation of the company's floors in the World Trade Center's South Tower after the North Tower was struck but did not escape before the South Tower collapsed. Timothy Maude, a U.S. Army lieutenant general and Army Deputy Chief of Staff, was the highest-ranking military officer killed at the Pentagon. Identification of remains continued for decades afterward; as of 2025, 1,103 of the World Trade Center victims, about 40 percent, remained unidentified.

That evening, the Central Intelligence Agency reported that its Counterterrorism Center had identified al-Qaeda as responsible for the attacks. The United States and allied forces subsequently invaded Afghanistan, where the Taliban government had allowed al-Qaeda to operate, and NATO invoked Article 5 of its founding treaty for the first time. Before the attacks, U.S. agencies had received fragmentary warnings; in July 2001, FBI agent Kenneth Williams sent a memo from Phoenix, Arizona, warning of a possible al-Qaeda effort to send operatives to American flight schools, but the warning was not acted on in time. Al-Qaeda's leadership initially denied involvement, then acknowledged responsibility in recorded statements, including one released in 2004. The manhunt for the group's leader ended in May 2011 with a U.S. military raid in Abbottabad, Pakistan.

Congress created the September 11th Victim Compensation Fund to compensate victims and their families in exchange for an agreement not to sue the airlines involved. Cleanup of the World Trade Center site finished in May 2002, and the Pentagon was repaired within a year. A replacement complex including the National September 11 Memorial & Museum was built at the World Trade Center site; its tallest building, One World Trade Center, opened in 2014. Additional memorials stand at the Pentagon and at the Flight 93 crash site in Pennsylvania.