9/11: What Happened on September 11, 2001, and What Changed After

A September morning

At 8:46 a.m. on Tuesday, September 11, 2001, New York time, a passenger jet struck the North Tower of the World Trade Center. It was the start of a sequence of attacks planned by the Islamist group al-Qaeda: within a little over an hour, four hijacked airliners were turned into weapons against targets in the United States. Nineteen hijackers were aboard the four planes in all: fifteen Saudi nationals, two from the United Arab Emirates, one from Egypt, Mohamed Atta, and one from Lebanon, Ziad Jarrah. Atta led the operatives on the ground; the operation’s tactical planner was Khalid Sheikh Mohammed; investigators later identified Osama bin Laden, al-Qaeda’s leader, as the mastermind.

The four flights

American Airlines 11, a Boeing bound from Boston to Los Angeles, took off at 7:59 a.m. with 92 people aboard, crew and passengers combined. At 8:14 a.m. came its last routine radio contact; sixteen seconds later air traffic control asked the plane to climb and got no response. At 8:19 a.m. a flight attendant alerted the airline that a hijacking was under way. About half an hour after takeoff, at 8:46 a.m., the plane struck the North Tower of the World Trade Center between floors 93 and 99.

United Airlines 175, a near-twin of the previous flight in route and scheduled departure time, also left Boston at 8:14 a.m. with 65 people aboard. It was hijacked around 8:42 a.m. and struck the South Tower, between floors 77 and 85, at 9:03 a.m.

American Airlines 77 took off at 8:20 a.m. from Washington Dulles bound for Los Angeles, with 64 people aboard. Its last routine radio contact came at 8:51 a.m.; between 8:51 and 8:54 a.m., as the plane flew over eastern Kentucky, the hijackers took control. Hani Hanjour, the only one of the five hijackers with a pilot’s license, flew it toward Washington. At 9:37:46 a.m. the plane struck the Pentagon at roughly 530 miles per hour, cutting through three of the building’s five concentric rings and destroying the load-bearing capacity of about 30 columns on the first floor.

United Airlines 93 left Newark at 8:42 a.m., about 45 minutes late because of air traffic, bound for San Francisco: 44 people aboard, four of them hijackers. Using the seatback phones and their own cellphones, some passengers learned what had happened on the other three flights. At 9:57 a.m. they voted to try to storm the cockpit. On a call with a telephone operator, passenger Todd Beamer was heard saying, “Are you ready. Okay. Let’s roll.” Facing the passengers’ attempt to break through the cockpit door, the hijackers deliberately put the plane into a dive, and it crashed in a field near Shanksville, Pennsylvania, at 10:03 a.m., at roughly 580 miles per hour. There were no survivors.

Practical example: Flight 93 is the only one of the four that never reached the target the hijackers had planned. Among the factors that set it apart was its late departure: by the time its passengers found out, over the phone, what had happened to the other three planes, they had enough time and information to decide to fight back. On the other three flights, hijacked before news of the attacks had spread, that chance never existed.

Later investigations, including the ones built on flight recordings recovered in the following months, made it possible to reconstruct the sequence of each of the four flights in precise detail; for more on how the instruments behind these reconstructions work, even after a plane is destroyed, see the article on the flight data recorder.

The collapse of the Twin Towers and World Trade Center 7

The South Tower fell at 9:59 a.m., 56 minutes after being struck. The North Tower, hit first, fell at 10:28 a.m., an hour and 42 minutes after impact. According to the technical investigation by NIST, the U.S. federal agency that led the official study of the collapse, the plane impacts severed structural support columns and stripped fireproofing insulation off the steel beams; fires on multiple floors, fed not only by jet fuel but also by office furniture and materials, reached temperatures of roughly 1,000°C. At that temperature steel’s strength drops to about 10% of its value at room temperature: the steel does not need to melt, and indeed NIST confirms that it did not, since its melting point is around 1,500°C.

At 5:20:52 p.m. that same day, World Trade Center 7 also collapsed — a 47-story building that had never been hit by any plane. Debris from the North Tower’s collapse had started uncontrolled fires on at least ten of its floors, while the sprinkler system in the affected area was not working. The heat weakened a critical support column, Column 79, until it failed: as the NIST report summarizes, “heating of floor beams and girders caused a critical support column to fail, initiating a fire-induced progressive collapse.” The building’s north face came down in roughly 5.4 seconds, about 40% slower than a free fall. It is the first known case worldwide of a tall building collapsing entirely because of fire, with no direct structural impact.

The victims, the response, the health effects

The total death toll from the attacks is 2,977, from 90 different countries: 2,753 in New York, 184 at the Pentagon, 40 on United Flight 93. At the Pentagon, 125 people died inside the building and 59 were aboard Flight 77; the section of the building that was hit collapsed about 30 minutes after impact, around 10:15 a.m.; the Pentagon’s own fire station responded immediately, and firefighters from Reagan National Airport and the Arlington County Fire Department arrived within minutes, helping save lives. In New York, 343 of the World Trade Center victims were firefighters; the remaining 2,410 were people who had been inside or around the buildings, including police officers.

In the days and months that followed, tens of thousands of people came to Ground Zero to help with the search and recovery effort, a job that went on for nearly nine months; about 300 trained dogs took part in the search. The last person pulled out alive from the rubble, Genelle Guzman-McMillan, who worked on the 64th floor of the North Tower, was rescued after 27 hours trapped beneath the debris.

The health effects lasted well beyond the day of the attacks: more than 400,000 people were exposed to the toxins released by the towers’ collapse and the prolonged fire, and according to the most recent figures reviewed in 2026, tens of thousands live with chronic respiratory illness, mental health conditions and more than 100 different types of cancer linked to that exposure; Congress funded treatment and compensation through the James Zadroga 9/11 Health and Compensation Act of 2010. More than two decades on, according to the most recent figures reviewed in 2026 from the New York City Office of Chief Medical Examiner, about 40% of World Trade Center victims — roughly 1,099 people — remained unidentified; identification continues using increasingly advanced forensic genetics.

Airspace closed for the first time in history

At 9:06 a.m. the FAA, the U.S. federal civil aviation agency, grounded all traffic not yet airborne that was bound for New York airspace. At 9:45 a.m. came the more drastic decision: the complete shutdown of United States airspace, the first time that had ever happened in the country’s history, with a matching measure coordinated in Canada. Within a few hours every commercial and private flight already in the air was brought down. Airports resumed operations gradually two days later, on September 13, 2001, under far tighter security checks than before.

The political aftermath and the official investigation

On September 20, 2001, President George W. Bush addressed Congress and declared war “on a radical network of terrorists and every government that supports them.” On October 7, 2001, U.S. military strikes began in Afghanistan, opening what would come to be called the war on terror. On October 26, 2001, Bush signed the USA PATRIOT Act, which expanded the government’s information-gathering powers and its ability to detain and deport immigrants suspected of terrorism. On November 25, 2002, the Homeland Security Act created the Department of Homeland Security: the largest reorganization of the U.S. federal government since the 1940s. On May 2, 2011, President Barack Obama announced the killing of Osama bin Laden in a raid by U.S. special forces in Abbottabad, Pakistan.

On July 22, 2004, the bipartisan commission set up by Congress, after more than 1,200 interviews across ten countries and a review of more than 2.5 million pages of documents, published its report: 585 pages, thirteen chapters. The Commission’s conclusion did not point to a single cause, but to a combination of failures across U.S. intelligence and security agencies: “a failure of imagination, policy, capabilities, and management.” Among the problems it identified: intelligence shortcomings, unsuccessful diplomatic efforts from 1997 to 2001, a lack of military options against the emerging terrorist threat, porous controls at the border and at airport security, and difficulty tracking terrorist financing. The Commission’s recommendations led, among other things, to the creation of the Director of National Intelligence and the National Counterterrorism Center.

Anyone wanting to trace the international backdrop the United States had lived through in the preceding decades can look to the Cold War, the confrontation that for more than forty years had shaped American foreign policy and security services before 2001; for the last precedent of a federal reorganization on this scale, the reference point is the Second World War.