On the morning of September 11, 2001, nineteen men from the Islamist group al-Qaeda took control of four American passenger airliners shortly after take-off and flew them into buildings. Two hit the towers of the World Trade Center in Lower Manhattan, one hit the Pentagon in Arlington, Virginia, and the fourth, United Airlines Flight 93, crashed into a field near Shanksville, Pennsylvania, after passengers tried to retake the cockpit. Nearly 3,000 died. It was more than in any other attack on the United States.
Two planes, two towers. American Airlines Flight 11 hit the North Tower at 8:46 in the morning. United Airlines Flight 175 hit the South Tower at 9:03. The South Tower, struck lower and with the aircraft travelling faster, collapsed at 9:59, after 56 minutes. The North Tower collapsed at 10:28, after 102 minutes. Both went down because the impact stripped the fireproofing from the steel floor trusses and the jet fuel fires heated them until they sagged and pulled the exterior columns in; the National Institute of Standards and Technology published that analysis in 2005. American Airlines Flight 77 hit the Pentagon at 9:37 and killed 125 people in the building along with everyone aboard.
Most of the dead were in the towers, and a large share of them were above the impact floors, where the stairwells were cut. Emergency services took heavy losses. In all, 343 New York City firefighters died, along with 23 police officers and 37 Port Authority officers. The hijackers had been trained in Afghanistan and Germany; the plot is generally dated to a meeting in 1999 and attributed to Khalid Sheikh Mohammed, with Osama bin Laden approving it.
The consequences were immediate and long. NATO invoked Article 5 for the first time in its history on September 12. The United States invaded Afghanistan on October 7, 2001, after the Taliban refused to hand over bin Laden. Congress passed the USA PATRIOT Act in October 2001, the Department of Homeland Security was created in 2002, and the US went to war in Iraq in 2003 on grounds that a later official inquiry found no connection to the attacks. Bin Laden was killed by American forces in Abbottabad, Pakistan, on May 2, 2011.
The site in Lower Manhattan holds a memorial with the names of the dead and a museum built around the surviving foundation walls. The last piece of the rebuilt complex, One World Trade Center, opened in 2014.