What happened on 9/11? The September 11 attacks explained

What happened on 9/11, 11 September 2001, was the deadliest terrorist attack in history. Nineteen men from the Islamist group al-Qaeda hijacked four passenger planes in the eastern United States. They flew two into the twin towers of the World Trade Center in New York and one into the Pentagon outside Washington. The fourth crashed in a Pennsylvania field after its passengers fought back. 2,977 people were killed, not counting the hijackers.

In short

• In 102 minutes, from the first crash at 8:46 a.m. to the fall of the second tower at 10:28 a.m., four planes were turned into weapons and the World Trade Center was destroyed.
• The attack was planned by al-Qaeda, led by Osama bin Laden, which had declared war on the United States in the 1990s.
• It led to the war in Afghanistan, new security and surveillance laws, and a "war on terror" that shaped US policy for two decades, including the contested decision to invade Iraq.

What happened, hour by hour?

All four planes were long-distance flights, chosen because they carried full fuel tanks. The hijackers, in teams of four or five, used knives and box cutters, claimed to have bombs, and took over the cockpits; four of them had trained as pilots. The times below are Eastern time, from the 9/11 Commission's report and the 9/11 Memorial & Museum.

Time	What happened
7:59 a.m.	American Airlines Flight 11 leaves Boston for Los Angeles; three more flights leave Boston, Washington Dulles and Newark by 8:42
8:46 a.m.	Flight 11 hits the North Tower of the World Trade Center, between floors 93 and 99
9:03 a.m.	United Airlines Flight 175 hits the South Tower, between floors 77 and 85, as millions watch on live TV
9:37 a.m.	American Airlines Flight 77 hits the west side of the Pentagon in Arlington, Virginia
9:42 a.m.	All planes over the United States are ordered to land
9:59 a.m.	The South Tower collapses, 56 minutes after it was hit
10:03 a.m.	United Airlines Flight 93 crashes near Shanksville, Pennsylvania, as passengers and crew try to retake the cockpit
10:28 a.m.	The North Tower collapses, 102 minutes after it was hit
5:20 p.m.	7 World Trade Center, a 47-story building set on fire by debris, collapses

On Flight 93, passengers learned by phone that other planes had been flown into buildings, and they decided to fight. The hijackers crashed the plane rather than lose control of it. The Commission concluded that its target was most likely the Capitol or the White House.

How many people died?

The 9/11 Memorial & Museum counts 2,977 people killed, from 90 countries: 2,753 in New York, 184 at the Pentagon and 40 aboard Flight 93. Between 16,400 and 18,000 people were in the World Trade Center complex when the first plane hit, and most of those below the impact floors got out before the towers fell. Above the impacts, the stairways were cut and almost no one escaped.

The dead in New York included 343 firefighters of the New York City Fire Department, who went into the towers as others came out, and dozens of police officers. The toll did not end that day. Rescue and clean-up workers at the site breathed dust full of crushed concrete, glass and asbestos, and in the years since, many have fallen ill and died of illnesses linked to it; a federal health program set up in 2011 monitors and treats them.

Who did it, and why?

The attacks were planned by al-Qaeda, a network founded by Osama bin Laden, a Saudi militant, and run in 2001 from Afghanistan, where the Taliban government sheltered it. The plan's chief architect was Khalid Sheikh Mohammed. Of the 19 hijackers, 15 were Saudi citizens, two came from the United Arab Emirates, one from Lebanon and one from Egypt.

Bin Laden had already declared war on the United States. In February 1998 he and four others issued what the Commission calls a "self-styled fatwa" saying Muslims should kill Americans, military or civilian, anywhere. His stated grievances included US troops in Saudi Arabia, US support for Israel and sanctions on Iraq. Al-Qaeda had bombed two US embassies in East Africa in 1998 and the warship USS Cole in 2000. The World Trade Center had been bombed before, in 1993, by a different group of extremists, killing six people.

The 9/11 Commission, set up by Congress and the president, reported in 2004 that warnings had been missed and agencies had failed to share what they knew. Its summary put it in one line: "The most important failure was one of imagination."

What changed after 9/11?

Within days, the country went on a war footing, and many of the changes are still in force:

• War in Afghanistan. On 18 September 2001, the president signed a joint resolution of Congress authorizing force against those responsible, a law still used to justify military action against terrorist groups. US-led strikes on Afghanistan began on 7 October 2001 and toppled the Taliban within weeks. US forces stayed for nearly 20 years and left in August 2021, when the Taliban returned to power.
• NATO's promise. On 12 September 2001, NATO invoked Article 5, its collective-defense clause, for the first and so far only time.
• Security at home. The USA PATRIOT Act of October 2001 widened the government's surveillance powers. The Transportation Security Administration took over airport screening, and the Department of Homeland Security was created in 2002.
• Bin Laden. He escaped in 2001 and was killed by US special forces in Pakistan on 2 May 2011.
• Iraq. In March 2003, the United States and its allies invaded Iraq, arguing that Saddam Hussein had weapons of mass destruction that might reach terrorists.

The attack also tested the limits of a president's war powers: the 2001 authorization was used for years against groups that did not exist on 9/11, which is one reason it is argued over in Congress.

Still argued

The Iraq War is the most contested consequence. Its supporters said that after 9/11 the risk of a hostile regime arming terrorists could not be tolerated. No stockpiles of weapons of mass destruction were found, and the 9/11 Commission found no evidence of a "collaborative operational relationship" between Iraq and al-Qaeda, so critics argue the war drew on 9/11 without being connected to it. A second long debate concerns the balance between security and liberty: surveillance programs, detention at Guantánamo Bay and harsh interrogation methods, which supporters defended as necessary and critics, including courts in some cases, found unlawful or wrong.

What is not argued among investigators is how the towers fell. The aircraft impacts cut columns and stripped fireproofing from the steel, and the huge fires that followed weakened it until the floors above came down. The U.S. National Institute of Standards and Technology found no evidence of controlled demolition, and conspiracy theories claiming otherwise have been examined and rejected.

How is 9/11 remembered?

For Americans who lived through it, 9/11 is a "where were you" day, often compared with the attack on Pearl Harbor in 1941; the 9/11 Commission notes that the toll of 2001 surpassed Pearl Harbor's. A memorial with two pools in the towers' footprints opened in New York in 2011, with the victims' names cut in bronze around them; there are memorials at the Pentagon and at Shanksville. Every year on 11 September, the names are read aloud.

For people born after 2001, it is history, and a turning point: the moment the post-Cold War decade ended and the "war on terror" began.

Check yourself

• Put the four flights in order, with where each one ended and roughly when.
• What did al-Qaeda say it wanted, and why was Afghanistan the first target of the US response?
• Why is the Iraq War argued over as a consequence of 9/11, when Iraq didn't carry out the attacks?

Sources

• Executive summary of the 9/11 Commission Report — National Commission on Terrorist Attacks Upon the United States, 2004
• The 9/11 Commission Report — National Commission on Terrorist Attacks Upon the United States, 2004
• 9/11 FAQs — National September 11 Memorial & Museum
• FAQs: NIST WTC towers investigation — National Institute of Standards and Technology
• Collective defence and Article 5 — NATO