September 11 attacks | History, Summary, Location, Timeline, Casualties, & Facts (2024)

September 11 attacks, series of airline hijackings and suicide attacks committed in 2001 by 19 militants associated with the Islamic extremist group al-Qaeda against targets in the United States, the deadliest terrorist attacks on American soil in U.S. history. The attacks against New York City and Washington, D.C., caused extensive death and destruction and triggered an enormous U.S. effort to combat terrorism. Some 2,750 people were killed in New York, 184 at the Pentagon, and 40 in Pennsylvania (where one of the hijacked planes crashed into the ground after the passengers attempted to retake the plane); all 19 terrorists died (see Researcher’s Note: September 11 attacks). Police and fire departments in New York were especially hard-hit: hundreds rushed to the scene of the attacks, and more than 400 police officers and firefighters were killed.

(Read Britannica’s interview with Jimmy Carter on 9/11 and world affairs.)

The plot

The September 11 attacks were precipitated in large part because Osama bin Laden, the leader of the militant Islamic organization al-Qaeda, held naive beliefs about the United States in the run-up to the attacks. Abu Walid al-Masri, an Egyptian who was a bin Laden associate in Afghanistan in the 1980s and ’90s, explained that, in the years prior to the attacks, bin Laden became increasingly convinced that America was weak. “He believed that the United States was much weaker than some of those around him thought,” Masri remembered, and “as evidence he referred to what happened to the United States in Beirut when the bombing of the Marines base led them to flee from Lebanon,” referring to the destruction of the marine barracks there in 1983 (see 1983 Beirut barracks bombings), which killed 241 American servicemen. Bin Laden believed that the United States was a “paper tiger,” a belief shaped not just by America’s departure from Lebanon following the marine barracks bombing but also by the withdrawal of American forces from Somalia in 1993, following the deaths of 18 U.S. servicemen in Mogadishu, and the American pullout from Vietnam in the 1970s.

The key operational planner of the September 11 attacks was Khalid Sheikh Mohammed (often referred to simply as “KSM” in the later 9/11 Commission Report and in the media), who had spent his youth in Kuwait. Khalid Sheikh Mohammed became active in the Muslim Brotherhood, which he joined at age 16, and then went to the United States to attend college, receiving a degree from North Carolina Agricultural and Technical State University in 1986. Afterward he traveled to Pakistan and then Afghanistan to wage jihad against the Soviet Union, which had launched an invasion against Afghanistan in 1979.

According to Yosri Fouda, a journalist at the Arabic-language cable television channel Al Jazeera who interviewed him in 2002, Khalid Sheikh Mohammed planned to blow up some dozen American planes in Asia during the mid-1990s, a plot (known as “Bojinka”) that failed, “but the dream of Khalid Sheikh Mohammed never faded. And I think by putting his hand in the hands of bin Laden, he realized that now he stood a chance of bringing about his long awaited dream.”

In 1996 Khalid Sheikh Mohammed met bin Laden in Tora Bora, Afghanistan. The 9-11 Commission (formally the National Commission on Terrorist Attacks Upon the United States), set up in 2002 by U.S. Pres. George W. Bush and the U.S. Congress to investigate the attacks of 2001, explained that it was then that Khalid Sheikh Mohammed “presented a proposal for an operation that would involve training pilots who would crash planes into buildings in the United States.” Khalid Sheikh Mohammed dreamed up the tactical innovation of using hijacked planes to attack the United States, al-Qaeda provided the personnel, money, and logistical support to execute the operation, and bin Laden wove the attacks on New York and Washington into a larger strategic framework of attacking the “far enemy”—the United States—in order to bring about regime change across the Middle East.

The September 11 plot demonstrated that al-Qaeda was an organization of global reach. The plot played out across the globe with planning meetings in Malaysia, operatives taking flight lessons in the United States, coordination by plot leaders based in Hamburg, Germany, money transfers from Dubai, and recruitment of suicide operatives from countries around the Middle East—all activities that were ultimately overseen by al-Qaeda’s leaders in Afghanistan.

Key parts of the September 11 plot took shape in Hamburg. Four of the key pilots and planners in the “Hamburg cell” who would take operational control of the September 11 attacks, including the lead hijacker Mohammed Atta, had a chance meeting on a train in Germany in 1999 with an Islamist militant who struck up a conversation with them about fighting jihad in the Russian republic of Chechnya. The militant put the Hamburg cell in touch with an al-Qaeda operative living in Germany who explained that it was difficult to get to Chechnya at that time, because many travelers were being detained in Georgia. He recommended they go to Afghanistan instead.

Although Afghanistan was critical to the rise of al-Qaeda, it was the experience that some of the plotters acquired in the West that made them simultaneously more zealous and better equipped to carry out the attacks. Three of the four plotters who would pilot the hijacked planes on September 11 and one of the key planners, Ramzi Binalshibh, became more radical while living in Hamburg. Some combination of perceived or real discrimination, alienation, and homesickness seems to have turned them all in a more militant direction. Increasingly cutting themselves off from the outside world, they gradually radicalized each other, and eventually the friends decided to wage battle in bin Laden’s global jihad, setting off for Afghanistan in 1999 in search of al-Qaeda.

Atta and the other members of the Hamburg group arrived in Afghanistan in 1999 right at the moment that the September 11 plot was beginning to take shape. Bin Laden and his military commander Muhammad Atef realized that Atta and his fellow Western-educated jihadists were far better suited to lead the attacks on Washington and New York than the men they had already recruited, leading bin Laden to appoint Atta to head the operation.

The hijackers, most of whom were from Saudi Arabia, established themselves in the United States, many well in advance of the attacks. They traveled in small groups, and some of them received commercial flight training.

Throughout his stay in the United States, Atta kept Binalshibh updated on the plot’s progress via e-mail. To cloak his activities, Atta wrote the messages as if he were writing to his girlfriend “Jenny,” using innocuous code to inform Binalshibh that they were almost complete in their training and readiness for the attacks. Atta wrote in one message, “The first semester commences in three weeks…Nineteen certificates for private education and four exams.” The referenced 19 “certificates” were code that identified the 19 al-Qaeda hijackers, while the four “exams” identified the targets of the attacks.

In the early morning of August 29, 2001, Atta called Binalshibh and said he had a riddle that he was trying to solve: “Two sticks, a dash and a cake with a stick down—what is it?” After considering the question, Binalshibh realized that Atta was telling him that the attacks would occur in two weeks—the two sticks being the number 11 and the cake with a stick down a 9. Putting it together, it meant that the attacks would occur on 11-9, or 11 September (in most countries the day precedes the month in numeric dates, but in the United States the month precedes the day; hence, it was 9-11 in the United States). On September 5 Binalshibh left Germany for Pakistan. Once there he sent a messenger to Afghanistan to inform bin Laden about both the day of the attack and its scope.