The September 11, 2001 attacks were a series of coordinated terrorist attacks carried out in the United States on September 11, 2001. Nineteen people affiliated to the al-Qaeda network, a militant Islamist organization, hijacked four commercial aircraft. They crashed one into each of the two tallest towers of the World Trade Center in Manhattan, New York City, shortly after which both towers collapsed. The third aircraft crashed into the U.S. Department of Defense headquarters, the Pentagon, in Arlington County, Virginia, just outside the capital, Washington, D.C.. The fourth plane crashed into a field near Shanksville, Pennsylvania following a passenger rebellion.
The attacks were the most lethal ever carried out in the U.S., and the first upon the mainland since the Japanese fire balloons of November 1944. The death toll of 2,986 exceeded the toll of approximately 2,400 dead after the surprise Japanese attack on Pearl Harbor on December 7, 1941.
The attacks involved the hijacking of four commercial airliners. With nearly 24,000 U.S. gallons (91,000 litres) of jet fuel aboard, the aircraft were turned into flying incendiary bombs. American Airlines Flight 11 crashed into the north side of the north tower of the World Trade Center (WTC) at 8:46:40 AM local time (12:46:40 UTC). At 9:03:11 AM local time (13:03:11 UTC), United Airlines Flight 175 crashed into the south tower, covered live on TV. American Airlines Flight 77 crashed into the Pentagon at 9:37:46 AM local time (13:37:46 UTC). The fourth hijacked plane, United Airlines Flight 93, crashed in a field near Shanksville and Stonycreek Township in Somerset County, Pennsylvania at either 10:03 AM local time (according to black box, radar, and cell-phone data) or 10:06 AM (according to faint seismic waves), with parts and debris found up to 8 miles away. The crash in Pennsylvania is believed to have resulted from the hijackers either deliberately crashing the aircraft or losing control of it as they fought with the passengers. No one in any of the hijacked aircraft survived.
The casualties were in the thousands: 265 on the planes; 2,595, including 343 firemen, in the WTC; and 125 at the Pentagon. At least 2,985 people were killed in total. In addition to the 110-floor Twin Towers of the WTC itself, five other buildings at the WTC site and four subway stations were destroyed or badly damaged. In total, on Manhattan Island, 25 buildings were damaged. Communications infrastructure such as broadcast radio, television and two way radio antenna towers were damaged beyond repair. In Arlington, a portion of the Pentagon was severely damaged by fire and one section of the building collapsed.
Lower Manhattan as seen from New Jersey
, shortly after the attacks
Some passengers and crew members were able to make phone calls from the doomed flights (see Communication during the September 11, 2001 attacks). They reported that multiple hijackers were aboard each plane. A total of 19 were later identified, four on United 93 and five each on the other three flights (though there have been some confusions over their exact names and photographs). The hijackers reportedly took control of the aircraft by using knives to kill flight attendants and at least one pilot or passenger. On American 77, one of the passengers reported that the hijackers used Leatherman utility knives.  Some form of noxious chemical spray, such as tear gas or pepper spray, was reported to have been used on American 11 and United 175 to keep passengers out of the first-class cabin. Bomb threats were made on three of the aircraft, but not on American 77.
Number of casualties
|World Trade Center
In New York, some of the occupants of each tower above its point of impact made their way upward towards the roof in hope of helicopter rescue. Because of the smoke and heat, no such rescue could safely be attempted. A number of people jumped from the burning building to the streets and roofs below. According to Associated Press, the city identified over 1,600 bodies but was unable to identify the rest of the bodies (about 1,100 people). They report that the city has "about 10,000 unidentified bone and tissue fragments that cannot be matched to the list of the dead" (AP, Feb 23, 2005).
The fourth aircraft
It has been speculated that the hijackers of the fourth hijacked aircraft, United Airlines Flight 93, intended to crash into the U.S. Capitol, the White House in Washington, DC, or Camp David. Black box recordings revealed that passengers attempted to seize control of the plane from the hijackers, who then rocked the plane in a failed attempt to subdue the passengers. Soon afterwards, the aircraft crashed in a field near Shanksville and Stonycreek Township in Pennsylvania at 10:03 AM (14:03 UTC) (according to black box and radar data) or 10:06 A.M. local time (14:06 UTC) (according to faint seismic waves). Captured al-Qaida mastermind Khalid Shaikh Mohammed is rumored to have said that Flight 93 was definitely targeting the Capitol.
Main article: Significance of the date of the September 11, 2001 attacks
The attacks are often referred to simply as September 11, 9/11, or 9-11. The latter two are from the U.S. style for writing short dates, and are pronounced "nine-eleven", though a few people say "nine-one-one" (the same as the telephone number for emergency services in the US, 911).
Main article: Responsibility for the September 11, 2001 attacks
The first public response from Osama bin Laden was read on September 16, 2001. He stated, "I stress that I have not carried out this act, which appears to have been carried out by individuals with their own motivation," which was broadcast by Qatar's Al-Jazeera satellite channel. (, , ). This denial went largely unremarked and often unreported in news media worldwide.
Osama bin Laden apparently took responsibility for the attacks on October 29, 2004, when he stated in a videotaped speech, sent to Al-Jazeera: "I say to you, Allah knows that it had never occurred to us to strike the towers. But after it became unbearable and we witnessed the oppression and tyranny of the American/Israeli coalition against our people in Palestine and Lebanon, it came to my mind. ... And as I looked at those demolished towers in Lebanon, it entered my mind that we should punish the oppressor in kind and that we should destroy towers in America in order that they taste some of what we tasted and so that they be deterred from killing our women and children." 
The militant Islamist al-Qaeda group had praised the attacks, and the group's leaders had previously hinted at their involvement in the incidents. Indeed, shortly after the attacks, the United States government declared al-Qaeda and bin Laden the prime suspects. The National Commission on Terrorist Attacks upon the United States, known as the 9-11 Commission, released its report on July 22, 2004, concluding that the attacks were conceived and implemented by al-Qaeda operatives. The commission reported that, while contacts with Iraq (along with several other Middle Eastern and African nations) had been made, it found no "collaborative relationship" between Iraq and al-Qaeda regarding the September 11 attacks.  The Commission also stated that "9/11 plotters eventually spent somewhere between $400,000 and $500,000 to plan and conduct their attack", but that the specific origin of the funds used to execute the attacks remained unknown.
Al-Qaeda had previously been involved in several attacks on American targets, notably the American embassies in Kenya and Tanzania. The September 11th attacks were consistent with their campaign against the United States, undertaken because of perceived American support for Israel's oppression of fellow Arab Muslims in Palestine, and American support for dictatorial regimes in the Middle East which al-Qaeda opposes (e.g. Saudi Arabia, Egypt, Tunisia, Yemen, and Jordan).
The book Al Qaeda: The true story of radical Islam by Jason Burke asserts that in the broader picture, Al-Qaeda's intent from the start was to unite nationalistic factions of militant Islam into a common world view, and to encourage and provide support for attacks against what it saw as the main enemies of a future Islamic world order.
In a 1998 fatwa, Bin Laden, Ayman al-Zawahiri, Abu-Yasir Rifa'i Ahmad Taha , Shaykh Mir Hamzah , and Fazlur Rahman declare the purpose of their jihad to be "[I]n order to liberate the al-Aqsa Mosque and the holy mosque [Mecca] from [the Americans'] grip, and in order for their armies to move out of all the lands of Islam, defeated and unable to threaten any Muslim."
The document also claims, "Third, if the Americans' aims behind these wars are religious and economic, the aim is also to serve the Jews' petty state and divert attention from its occupation of Jerusalem and murder of Muslims there. The best proof of this is their eagerness to destroy Iraq, the strongest neighboring Arab state, and their endeavor to fragment all the states of the region such as Iraq, Saudi Arabia, Egypt, and Sudan into paper statelets and through their disunion and weakness to guarantee Israel's survival and the continuation of the brutal crusade occupation of the Peninsula." 
Main article: Aftermath of the September 11, 2001 attacks
Military and security measures
The attacks defined the first term of President George W. Bush and led to what he has called the War on Terror, or war against terrorism. The accuracy of describing it as a "war" and the political motivations and consequences are the topic of strenuous debate. The U.S. government increased military operations, economic measures and political pressure on groups it accused of being terrorists, as well as on governments and countries accused of sheltering them. October 2001 saw the first military action initiated by the U.S. under this policy, when the U.S. invaded Afghanistan in a failed attempt to capture Osama bin Laden. Prior to the invasion, the Taliban had refused to hand over Osama bin Laden without being shown evidence of his connection to the attacks. While the primary objective of capturing Osama bin Laden failed, the invasion did succeed in removing the Taliban from power, enabling the implementation of a government somewhat more cooperative and supportive in the search for Osama bin Laden and the general "War on Terror".
The attacks had major world-wide political effects. The attacks were denounced world-wide and approximately one month after the attacks the USA led a wide coalition of international forces into Afghanistan in pursuit of al-Qaeda forces. Many countries introduced tough anti-terrorism legislation and took action to cut off terrorist finances, including the freezing of bank accounts suspected of being used to fund terrorism. Law enforcement and intelligence agencies stepped up cooperation to arrest terrorist suspects and break up suspected terrorist cells around the world. This process was highly controversial, as restrictions on government authority were lifted and certain civil rights protections were rescinded. The controversy was highlighted in September 2004 when Yusuf Islam, a leading British Muslim noted for his peaceful charitable work and previously known as the singer Cat Stevens, was barred from entering the U.S. and was subsequently returned to the UK after his flight was briefly diverted to Maine. Yusuf Islam's expulsion led to a complaint from British foreign secretary, Jack Straw to the U.S. Secretary of State Colin Powell, who ordered a review of restrictions placed on people entering the United States.
Reaction amongst the United States population
The attacks also had immediate and overwhelming effects upon the United States population. Gratitude toward uniformed public-safety workers, and especially toward firefighters, was widely expressed in light of both the drama of the risks taken on the scene and the high death toll among the workers. The number of casualties among the emergency service personnel was unprecedented. The highly visible role played by Rudolph Giuliani, the Mayor of New York City, won him high praise nationally. He was named Person of the Year by Time magazine for 2001, and at times had a higher profile in the U.S. than President George W. Bush. In 2004, the Zogby poll showed that over half of New Yorkers believed that the government performed some kind of cover-up operation and that there was major government involvement.
Two other major public reactions to the attacks were a surge in patriotism and flag-waving not seen since World War II, and an unprecedented level of respect, sympathy, and admiration for New York City and New Yorkers as a group by Americans from other parts of the U.S. Some criticized this particular reaction, noting that not everyone who died was from New York (for example, some of the passengers on the planes). However, New York City clearly bore the brunt of the attacks -- and for years to come will still bear physical scars from the events of that day.
The attacks had significant economic repercussions for the United States and world markets. The New York Stock Exchange, the American Stock Exchange and NASDAQ did not open on September 11 and remained closed until September 17. New York Stock Exchange (“NYSE”) facilities and remote data processing sites were not damaged by the attack, but member firms, customers and markets were unable to communicate due to major damage to the telephone exchange facility near the World Trade Center. When the stock markets reopened on September 17, 2001, after the longest closure since the Great Depression in 1933, the Dow Jones Industrial Average (“DJIA”) stock market index fell 684 points, or 7.1%, to 8920, its biggest-ever one-day point decline. By the end of the week, the DJIA had fallen 1369.7 points (14.3%), its largest one-week point drop in history. U.S. stocks lost $1.2 trillion in value for the week. As of 2005 the streets surrounding the Stock Exchange on Wall Street are still barricaded to prevent a physical attack upon the building.
North American air space was closed for several days after the attacks and air travel decreased significantly upon its reopening. As of 2005, the U.S. airline industry has not fully recovered.
Rescue and recovery
Rescue and recovery efforts took months to complete. It took several weeks to simply put out the fires burning in the rubble of the WTC, and the clean-up was not completed until May 2002. Many relief funds were immediately set up to assist victims of the attacks. The task of providing financial assistance to the survivors and the families of victims is still ongoing.
Collapse of the World Trade Center
Main article: Collapse of the World Trade Center
: A New York City firefighter looks up at what remains of the South Tower.
Buildings surrounding the World Trade Center were heavily damaged by the debris and massive force of the falling twin towers.
There has been much speculation on the "performance" of the Twin Towers after the impacts, and the reasons for the collapse are under active debate by structural engineers, architects and the relevant U.S. government agencies. The design of the WTC included many basic innovations distinguishing it from all previous skyscrapers and from many built since. Although the kinetic force of the jetliner hits and the resulting conflagrations were unprecedented in the history of building disasters, some engineers strongly believe skyscrapers of more traditional design (such as New York City's Empire State Building and Malaysia's Petronas Towers) would have fared much better under the circumstances, perhaps standing indefinitely. If they are correct, supertall buildings that share the WTC's major design elements (for example, Chicago's Sears Tower and John Hancock Center) could be considered particularly vulnerable.
7 World Trade Center collapsed in the late afternoon of September 11. It was hit by falling debris of the Twin Towers; for details on its collapse see: Destruction of 7 World Trade Center.
A federal technical building and fire safety investigation of the collapses of the Twin Towers and 7 WTC has been conducted by the United States Department of Commerce's National Institute of Standards and Technology (NIST). The goals of this investigation -- completed on April 6th 2005 -- were to investigate the building construction, the materials used, and the technical conditions that contributed to the outcome of the WTC disaster. The investigation  will serve as the basis for:
- Improvements in the way buildings are designed, constructed, maintained, and used
- Improved tools and guidance for industry and safety officials
- Revisions to building and fire codes, standards, and practices
- Improved public safety
The long anticipated report on the collapse by the National Institute of Standards and Technology (NIST) was released on April 6, 2005. In its over 10,000 pages the conclusion reached was that the fireproofing on the steel infrastructure was blown off by the initial impact of the planes into the towers. If this had not occurred the WTC would have likely remained standing. Furthermore the staircases were not adequately reinforced to provide emergency escape for people above the impact zone.
See Survivors of the September 11, 2001 attacks
Intelligence, enquiries and fact-finding
Main article: 9/11 Commission Report
Speculation and claims of further conspiracies
Main article: 9/11 conspiracy theories | See also: 9/11 domestic conspiracy theory| 9/11 conspiracy claims regarding Jews or Israel. Since the attacks, there has been much speculation concerning their planning, especially whether more attacks were planned.
Twenty-seven members of al-Qaida attempted to enter the United States to take part in the September 11 attacks. In the end, only nineteen participated. Other would-be hijackers are often referred to as the 20th hijackers.
Ramzi Binalshibh meant to take part in the attacks, but he was repeatedly denied a visa for entry into the U.S. Mohamed al-Kahtani, a Saudi Arabian citizen, may also have been planning to join the hijackers but U.S. Immigration authorities at Orlando International Airport refused his entry into the U.S. in August 2001. He was later captured in Afghanistan and imprisoned at the U.S. military prison known as Camp X-ray at Guantanamo Bay, Cuba. Zacarias Moussaoui was reportedly considered as a replacement for Ziad Jarrah, who at one point threatened to withdraw from the scheme because of tensions amongst the plotters. Plans to include Moussaoui were never completed because the al-Qaida hierarchy had doubts about his reliability. Ultimately, Moussaoui did not play a role in the hijacking.
Other al-Qaida members who may have attempted, but were unable, to take part in the attacks include Saeed al-Ghamdi (not to be confused with the successful hijacker of the same name), Mushabib al-Hamlan, Zakariyah Essabar, Ali Abdul Aziz Ali, and Tawfiq bin Attash. According to the 9/11 Commission Report, Khalid Sheikh Mohammed, the attack's mastermind, wanted to remove at least one member — Khalid al-Mihdhar — from the operation, but he was overruled by Osama bin Laden.
- Further reading