The September 11 attacks (often referred to as September 11th or 9/11) were a series of coordinated suicide attacks by Al-Qaeda upon the United States on September 11, 2001. On that morning, 19 Al-Qaeda terrorists hijacked four commercial passenger jet airliners.The hijackers intentionally crashed two of the airliners into the Twin Towers of the World Trade Center in New York City, killing everyone on board and many others working in the buildings. Both buildings collapsed within two hours, destroying nearby buildings and damaging others. The hijackers crashed a third airliner into the Pentagon in Arlington, Virginia, just outside of Washington, D.C. The fourth plane crashed into a field near Shanksville in rural Pennsylvania, after some of its passengers and flight crew attempted to retake control of the plane, which the hijackers had redirected toward Washington, D.C. There were no survivors from any of the flights.

In total 2,993 people, including the hijackers, died in the attacks.The overwhelming majority of casualties were civilians, including nationals of over 90 countries. In addition, the death of at least one person from lung disease was ruled by a medical examiner to be a result of exposure to dust from the World Trade Center's collapse.[5] The United States responded to the attacks by launching a "War on Terrorism", invading Afghanistan to depose the Taliban, who had harbored al-Qaeda terrorists, and enacting the USA PATRIOT Act. Many other countries also strengthened their anti-terrorism legislation and expanded law enforcement powers. Some American stock exchanges stayed closed for the rest of the week following the attack, and posted enormous losses upon reopening, especially in the airline and insurance industries. The destruction of billions of dollars worth of office space caused serious damage to the economy of Lower Manhattan.

The damage to the Pentagon was cleared and repaired within a year, and the Pentagon Memorial was built on the site. The rebuilding process has started on the World Trade Center site. In 2006 a new office tower was completed on the site of 7 World Trade Center. The 1 World Trade Center is currently under construction at the site and at 1,776 ft (541 m) upon completion in 2011, will become one of the tallest buildings in North America. Three more towers were originally expected to be built between 2007 and 2012 on the site.
See also: Responsibility for the September 11 attacks, Hijackers in the September 11 attacks, Trials related to the September 11 attacks, and 20th hijacker

Within hours of the attacks, the FBI was able to determine the names and in many cases the personal details of the suspected pilots and hijackers. Mohamed Atta's luggage, which did not make the connection from his Portland flight onto Flight 11, contained papers that revealed the identity of all 19 hijackers (all men), and other important clues about their plans, motives, and backgrounds. On the day of the attacks, the National Security Agency intercepted communications that pointed to Osama bin Laden, as did German intelligence agencies. On September 27, 2001, the FBI released photos of the 19 hijackers, along with information about the possible nationalities and aliases of many. Fifteen of the hijackers were from Saudi Arabia, two from the United Arab Emirates, one from Egypt, and one from Lebanon.Mohamed Atta was the ringleader of the 19 hijackers. The hijackers were well-educated, mature adults, whose belief systems were fully formed.

The FBI investigation into the attacks, code named operation PENTTBOM, was the largest and most complex investigation in the history of the FBI, involving over 7,000 special agents. Through interrogations of USS Cole bombing suspects in Yemen, the FBI was able to link the hijackers to al-Qaeda.The United States government determined that al-Qaeda, headed by Osama bin Laden, bore responsibility for the attacks, with the FBI stating "evidence linking al-Qaeda and bin Laden to the attacks of September 11 is clear and irrefutable". The Government of the United Kingdom reached the same conclusion regarding al-Qaeda and Osama bin Laden's culpability for the September 11 attacks.[84]

Author Laurie Mylroie writing in the conservative political magazine The American Spectator in 2006 argues that Khalid Sheikh Mohammed and his family are the primary architects of 9/11 and similar attacks, and that Khalid Sheikh Mohammed's association with Osama bin Laden is secondary and that Al-Qaeda's claim of responsibilty for the attack is after the fact and opportunistic.[85] In an opposing point of view, former CIA officer Robert Baer, writing in Time magazine in 2007, asserts that George W. Bush Adminstration's publicizing of Khalid Sheikh Mohammed's claims of responsibility for 9/11 and numerous other acts was a mendacious attempt to claim that all of the significant actors in 9/11 had been caught.

Al-Qaeda

Main article: Al-Qaeda

The origins of al-Qaeda can be traced back to 1979 when the Soviet Union invaded Afghanistan. Soon after the invasion, Osama bin Laden traveled to Afghanistan where, with American government assistance,[87] he helped organize Arab mujahideen and established the Maktab al-Khidamat (MAK) organization to resist the Soviets. In 1989, as the Soviets withdrew, MAK was transformed into a "rapid reaction force" in jihad against governments across the Muslim world. Under the guidance of Ayman al-Zawahiri, Osama bin Laden became more radical.[88] In 1996, bin Laden issued his first fatwā, which called for American soldiers to leave Saudi Arabia.[89]

In a second fatwā issued in 1998, bin Laden outlined his objections to American foreign policy towards Israel, as well as the continued presence of American troops in Saudi Arabia after the Gulf War Bin Laden used Islamic texts to exhort violent action against American military and citizenry until the stated grievances are reversed, noting "ulema have throughout Islamic history unanimously agreed that the jihad is an individual duty if the enemy destroys the Muslim countries."