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The destruction of the World Trade Center and its environs seriously harmed the U.S. economy and induced global market shocks.

Many other countries strengthened anti-terrorism legislation and expanded their powers of law enforcement and intelligence agencies.

Cleanup of the World Trade Center site (colloquially "Ground Zero") took eight months and was completed in May 2002, while the Pentagon was repaired within a year.

After delays in the design of a replacement complex, construction of the One World Trade Center began in November 2006; it opened in November 2014.

Memorials to the attacks include the National September 11 Memorial & Museum in New York City, The Pentagon Memorial in Arlington County, Virginia, and the Flight 93 National Memorial at the Pennsylvania crash site.

Al-Qaeda's origins can be traced to 1979, when the Soviet Union invaded Afghanistan.[13] Soon after the invasion, Osama bin Laden traveled to Afghanistan and helped organize Arab mujahideen (the "Afghan Arabs") to resist the "Communist invaders" (Soviets) until their exit from the country in 1989.[14][15] In 1984 bin Laden, along with Islamic scholar Abdullah Azzam, formed the Maktab al-Khidamat (MAK), an organization to support Arab mujahideen who came to join the jihad in Afghanistan.[13][16] The U.S. Central Intelligence Agency (CIA) funneled several billion dollars worth of weapons to the indigenous Afghan mujahideen resistance, a portion of which bled to the Arab volunteers.[17] However, no direct evidence of U.S. aid to bin Laden or any of his affiliates was ever uncovered.[18] In 1996, bin Laden issued his first fatwā, which declared war against the United States and demanded the expulsion of all American soldiers from the Arabian Peninsula.[19] In a second 1998 fatwā, bin Laden outlined his objections to American foreign policy with respect to the State of Israel, as well as the continued presence of American troops in Saudi Arabia after the Gulf War.[20] Bin Laden maintained that Muslims are obliged to attack American targets until the aggressive policies of the U.S. against Muslims were reversed.

According to bin Laden, Islamic jurists had "throughout Islamic history unanimously agreed that the jihad is an individual duty if the enemy destroys the Muslim countries".[20][21] The Hamburg cell in Germany included Islamists who eventually came to be key operatives in the 9/11 attacks.[22] Mohamed Atta; Marwan al-Shehhi; Ziad Jarrah; Ramzi bin al-Shibh; and Said Bahaji were all members of al-Qaeda's Hamburg cell.[23] Bin Laden asserted that all Muslims have a duty to wage defensive war against the United States, and combat American aggression.

He further argued that military strikes against American assets would send a message to the American people, attempting to force the U.S. to re-evaluate its support to Israel, and other aggressive policies.[24] In a 1998 interview with American journalist John Miller, bin Laden stated: [W]e tell the Americans as people and we tell the mothers of soldiers and American mothers in general that if they value their lives and the lives of their children, to find a nationalistic government that will look after their interests and not the interests of the Jews.

The continuation of tyranny will bring the fight to America, as [the 1993 World Trade Center bomber] Ramzi [Yousef] yourself and others did.

This is my message to the American people: to look for a serious government that looks out for their interests and does not attack others, their lands, or their honor.

And my word to American journalists is not to ask why we did that but ask what their government has done that forced us to defend ourselves.

Bin Laden orchestrated the September 11 attacks.

He initially denied involvement, but later recanted his denial.[26][27][28] Al Jazeera broadcast a statement by him on September 16, 2001: "I stress that I have not carried out this act, which appears to have been carried out by individuals with their own motivation".[29] In November 2001, U.S. forces recovered a videotape from a destroyed house in Jalalabad, Afghanistan.

In the video, bin Laden, talking to Khaled al-Harbi, admitted foreknowledge of the attacks.[30] On December 27, 2001, a second video of bin Laden was released in which he, stopping short of admitting responsibility for the attacks, said:[31] It has become clear that the West in general and America in particular have an unspeakable hatred for Islam.

...

It is the hatred of crusaders.

Terrorism against America deserves to be praised because it was a response to injustice, aimed at forcing America to stop its support for Israel, which kills our people.

...

We say that the end of the United States is imminent, whether Bin Laden or his followers are alive or dead, for the awakening of the Muslim ummah [nation] has occurred.

...

It is important to hit the economy (of the United States), which is the base of its military power...If the economy is hit they will become reoccupied.

Shortly before the 2004 U.S. presidential election, bin Laden used a taped statement to publicly acknowledge al-Qaeda's involvement in the attacks.[26] He admitted his direct link to the attacks and said they were carried out because: The events that affected my soul in a direct way started in 1982 when America permitted the Israelis to invade Lebanon and the American Sixth Fleet helped them in that.

This bombardment began and many were killed and injured and others were terrorised and displaced.

I couldn't forget those moving scenes, blood and severed limbs, women and children sprawled everywhere.

Houses destroyed along with their occupants and high rises demolished over their residents, rockets raining down on our home without mercy...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.

And that day, it was confirmed to me that oppression and the intentional killing of innocent women and children is a deliberate American policy.

Destruction is freedom and democracy, while resistance is terrorism and intolerance.[32] Bin Laden personally directed his followers to attack the World Trade Center and the Pentagon.[33][34] Another video obtained by Al Jazeera in September 2006 showed bin Laden with one of the attacks' chief planners, Ramzi bin al-Shibh, as well as hijackers, Hamza al-Ghamdi and Wail al-Shehri, amidst making preparations for the attacks.[35] Bin Laden had been on the FBI's Most Wanted List since 1998 for the 1998 bombings of the U.S. embassies in Tanzania and Kenya.[36][37] Journalist Yosri Fouda of the Arabic television channel Al Jazeera reported that in April 2002 al-Qaeda member Khalid Sheikh Mohammed admitted his involvement in the attacks, along with Ramzi bin al-Shibh.[38][39][40] The 2004 9/11 Commission Report determined that Mohammed's animosity towards the United States, the principal architect of the 9/11 attacks, stemmed from his "violent disagreement with U.S. foreign policy favoring Israel".[41] Mohammed was also an adviser and financier of the 1993 World Trade Center bombing and the uncle of Ramzi Yousef, the lead bomber in that attack.[42][43] In late 1994, Mohammed and Yousef moved on to plan a new terrorist attack called the Bojinka plot planned for January 1995.

Despite a failure and Yousef's capture by U.S. forces the following month, the Bojinka plot would influence the later 9/11 attacks.[44] In "Substitution for Testimony of Khalid Sheikh Mohammed" from the trial of Zacarias Moussaoui, five people are identified as having been completely aware of the operation's details.

They are bin Laden; Khalid Sheikh Mohammed; Ramzi bin al-Shibh; Abu Turab al-Urduni; and Mohammed Atef.[45] Osama bin Laden's declaration of a holy war against the United States, and a 1998 fatwā signed by bin Laden and others that called for the killing of Americans,[20][46] are seen by investigators as evidence of his motivation.[47] During his interview with Hamid Mir in November 2001, Bin Laden defended the September 11 attacks as retaliatory strikes against American atrocities against Muslims across the world.

He also maintained that the attacks were not directed against women and children, asserting that the targets of the strikes were symbols of America's "economic and military power".[48][49] In bin Laden's November 2002 "Letter to the American people", he explicitly stated that al-Qaeda's motives for the attacks included: After the attacks, bin Laden and Ayman al-Zawahiri released additional videotapes and audio recordings, some of which repeated the above reasons for the attacks.

Two relevant publications were bin Laden's 2002 "Letter to the American people"[60] and a 2004 videotape by bin Laden.[61] [...] those young men, for whom God has cleared the way, didn't set out to kill children, but rather attacked the biggest center of military power in the world, the Pentagon, which contains more than 64,000 workers, a military base which has a big concentration of army and intelligence ...

As for the World Trade Center, the ones who were attacked and who died in it were part of a financial power.

It wasn't a children's school! Neither was it a residence.

And the general consensus is that most of the people who were in the towers were men that backed the biggest financial force in the world, which spreads mischief throughout the world.

— Osama Bin Laden's interview with Tayseer Allouni, 21 October 2001[62] As an adherent of Islam, bin Laden believed that non-Muslims are forbidden from having a permanent presence in the Arabian Peninsula.[63] In 1996, bin Laden issued a fatwā calling for American troops to leave Saudi Arabia.

In 1998, al-Qaeda wrote "for over seven years the United States has been occupying the lands of Islam in the holiest of places, the Arabian Peninsula, plundering its riches, dictating to its rulers, humiliating its people, terrorizing its neighbors, and turning its bases in the Peninsula into a spearhead through which to fight the neighboring Muslim peoples".[64] In a December 1999 interview, bin Laden said he felt that Americans were "too near to Mecca", and considered this a provocation to the entire Muslim world.[65] One analysis of suicide terrorism suggested that without U.S. troops in Saudi Arabia, al-Qaeda likely would not have been able to get people to commit to suicide missions.[66] In the 1998 fatwā, al-Qaeda identified the Iraq sanctions as a reason to kill Americans, condemning the "protracted blockade" among other actions that constitute a declaration of war against "Allah, his messenger, and Muslims".[64] The fatwā declared that "the ruling to kill the Americans and their allies – civilians and military – is an individual duty for every Muslim who can do it in any country in which it is possible to do it, in order to liberate the al-Aqsa Mosque and the holy mosque of Mecca from their grip, and in order for their [the Americans'] armies to move out of all the lands of Islam, defeated and unable to threaten any Muslim".[20][67] In 2004, bin Laden claimed that the idea of destroying the towers had first occurred to him in 1982, when he witnessed Israel's bombardment of high-rise apartment buildings during the 1982 Lebanon War.[68][69] Some analysts, including political scientists John Mearsheimer and Stephen Walt, also claimed that U.S. support of Israel was a motive for the attacks.[51][65] In 2004 and 2010, bin Laden again connected the September 11 attacks with U.S. support of Israel, although most of the letter expressed bin Laden's disdain for President Bush and bin Laden's hope to "destroy and bankrupt" the U.S.[70][71] Other motives have been suggested in addition to those stated by bin Laden and al-Qaeda.

Some authors suggested the "humiliation" that resulted from the Islamic world falling behind the Western world – this discrepancy was rendered especially visible by globalization[72][73] and a desire to provoke the U.S. into a broader war against the Islamic world in the hope of motivating more allies to support al-Qaeda.

Similarly, others have argued the 9/11 attacks were a strategic move to provoke America into a war that would incite a pan-Islamic revolution.[74][75] Documents seized during the 2011 operation that killed bin Laden included a few notes handwritten by bin Laden in September 2002 with the heading "The Birth of the Idea of September 11".

In these notes he describes how he was inspired by the crash of EgyptAir Flight 990 on October 31, 1999, which was deliberately crashed by co-pilot Gameel Al-Batouti.

"This is how the idea of 9/11 was conceived and developed in my head, and that is when we began the planning" bin Laden continued, adding that no one but Abu Hafs and Abu al-Khair knew about it at the time.

The 9/11 Commission Report identified Khalid Sheikh Mohammed as the architect of 9/11, but he is not mentioned in bin Laden's notes.[76] The attacks were conceived by Khalid Sheikh Mohammed, who first presented it to Osama bin Laden in 1996.[77] At that time, bin Laden and al-Qaeda were in a period of transition, having just relocated back to Afghanistan from Sudan.[78] The 1998 African embassy bombings and bin Laden's February 1998 fatwā marked a turning point of al-Qaeda's terrorist operation,[79] as bin Laden became intent on attacking the United States.

In late 1998 or early 1999, bin Laden gave approval for Mohammed to go forward with organizing the plot.[80] Mohammed, bin Laden, and Mohammed Atef, bin Laden's deputy, held a series of meetings in early 1999.[81] Atef provided operational support, including target selections and helping arrange travel for the hijackers.[78] Bin Laden overruled Mohammed, rejecting potential targets such as the U.S. Bank Tower in Los Angeles for lack of time.[82][83] Bin Laden provided leadership and financial support and was involved in selecting participants.[84] He initially selected Nawaf al-Hazmi and Khalid al-Mihdhar, both experienced jihadists who had fought in Bosnia.

Hazmi and Mihdhar arrived in the United States in mid-January 2000.

In early 2000, Hazmi and Mihdhar took flying lessons in San Diego, California.

Both spoke little English, performed poorly in flying lessons, and eventually served as secondary "muscle" hijackers.[85][86] In late 1999, a group of men from Hamburg, Germany, arrived in Afghanistan.

The group included Mohamed Atta, Marwan al-Shehhi, Ziad Jarrah, and Ramzi bin al-Shibh.[87] Bin Laden selected these men because they were educated, could speak English, and had experience living in the West.[88] New recruits were routinely screened for special skills and al-Qaeda leaders consequently discovered that Hani Hanjour already had a commercial pilot's license.[89] Mohammed later said that he helped the hijackers blend in by teaching them how to order food in restaurants and dress in Western clothing.[90] Hanjour arrived in San Diego on December 8, 2000, joining Hazmi.[91]: 6–7 They soon left for Arizona, where Hanjour took refresher training.[91]: 7 Marwan al-Shehhi arrived at the end of May 2000, while Atta arrived on June 3, 2000, and Jarrah arrived on June 27, 2000.[91]: 6 Bin al-Shibh applied several times for a visa to the United States, but as a Yemeni, he was rejected out of concerns he would overstay his visa.[91]: 4, 14 Bin al-Shibh stayed in Hamburg, providing coordination between Atta and Mohammed.[91]: 16 The three Hamburg cell members all took pilot training in South Florida at Huffman Aviation.[91]: 6 In the spring of 2001, the secondary hijackers began arriving in the United States.[92] In July 2001, Atta met with bin al-Shibh in Tarragona, Catalonia, Spain, where they coordinated details of the plot, including final target selection.

Bin al-Shibh also passed along bin Laden's wish for the attacks to be carried out as soon as possible.[93] Some of the hijackers received passports from corrupt Saudi officials who were family members or used fraudulent passports to gain entry.[94] There have been a few theories that 9/11 was selected by the hijackers as the date of the attack because of its resemblance to 9-1-1, the phone number used to report emergencies in the United States.

However, Lawrence Wright wrote that the hijackers chose the date when John III Sobieski, the King of Poland and Grand Duke of Lithuania, began the battle that turned back the Ottoman Empire's Muslim armies that were attempting to capture Vienna (present-day capital of Austria) on 11 September 1683.

During 1683, Vienna was the seat of the Holy Roman Empire and Habsburg monarchy, both major powers in Europe at the time.

For Osama bin Laden, this was a date when the West gained some dominance over Islam, and by attacking on this date, he hoped to make a step in Islam "winning" the war for worldwide power and influence.[95] In late 1999, al-Qaeda associate Walid bin Attash ("Khallad") contacted Mihdhar and told him to meet in Kuala Lumpur, Malaysia; Hazmi and Abu Bara al Yemeni would also be in attendance.

The NSA intercepted a telephone call mentioning the meeting, Mihdhar, and the name "Nawaf" (Hazmi); while the agency feared "Something nefarious might be afoot", it took no further action.

The CIA had already been alerted by Saudi intelligence about the status of Mihdhar and Hazmi as al-Qaeda members, and a CIA team broke into Mihdhar's Dubai hotel room and discovered that Mihdhar had a U.S. visa.

While Alec Station alerted intelligence agencies worldwide about this fact, it did not share this information with the FBI.

The Malaysian Special Branch observed the January 5, 2000, meeting of the two al-Qaeda members and informed the CIA that Mihdhar, Hazmi, and Khallad were flying to Bangkok, but the CIA never notified other agencies of this, nor did it ask the State Department to put Mihdhar on its watchlist.

An FBI liaison to Alec Station asked permission to inform the FBI of the meeting but was told: "This is not a matter for the FBI".[96] By late June, senior counter-terrorism official Richard Clarke and CIA director George Tenet were "convinced that a major series of attacks was about to come", although the CIA believed the attacks would likely occur in Saudi Arabia or Israel.[97] In early July, Clarke put domestic agencies on "full alert", telling them "Something really spectacular is going to happen here.

soon".

He asked the FBI and the State Department to alert the embassies and police departments, and the Defense Department to go to "Threat Condition Delta".[98][99] Clarke later wrote: "Somewhere in CIA there was information that two known al Qaeda terrorists had come into the United States.

Somewhere in FBI, there was information that strange things had been going on at flight schools in the United States ...

They had specific information about individual terrorists from which one could have deduced what was about to happen.

None of that information got to me or the White House".[100] [...] by July [2001], with word spreading of a coming attack, a schism emerged among the senior leadership of al Qaeda.

Several senior members reportedly agreed with Mullah Omar.

Those who reportedly sided with bin Ladin included Atef, Sulayman Abu Ghayth, and KSM.

But those said to have opposed him were weighty figures in the organization-including Abu Hafs the Mauritanian, Sheikh Saeed al Masri, and Sayf al Adl.

One senior al Qaeda operative claims to recall Bin Ladin arguing that attacks against the United States needed to be carried out immediately to support insurgency in the Israeli-occupied territories and protest the presence of U.S. forces in Saudi Arabia.

— 9/11 Commission Report, pp.

251[101] On July 13, Tom Wilshire, a CIA agent assigned to the FBI's international terrorism division, emailed his superiors at the CIA's Counterterrorism Center (CTC) requesting permission to inform the FBI that Hazmi was in the country and that Mihdhar had a U.S. visa.

The CIA never responded.[102] The same day in July, Margarette Gillespie, an FBI analyst working in the CTC, was told to review material about the Malaysia meeting.

She was not told of the participants' presence in the U.S. The CIA gave Gillespie surveillance photos of Mihdhar and Hazmi from the meeting to show to FBI counterterrorism, but did not tell her their significance.

The Intelink database informed her not to share intelligence material on the meeting with criminal investigators.

When shown the photos, the FBI were refused more details on their significance, and they were not given Mihdhar's date of birth nor passport number.[103] In late August 2001, Gillespie told the INS, the State Department, the Customs Service, and the FBI to put Hazmi and Mihdhar on their watchlists, but the FBI was prohibited from using criminal agents in searching for the duo, hindering their efforts.[104] Also in July, a Phoenix-based FBI agent sent a message to FBI headquarters, Alec Station, and FBI agents in New York alerting them to "the possibility of a coordinated effort by Osama bin Laden to send students to the United States to attend civil aviation universities and colleges".

The agent, Kenneth Williams, suggested the need to interview all flight school managers and identify all Arab students seeking flight training.[105] In July, Jordan alerted the U.S. that al-Qaeda was planning an attack on the U.S.; "months later", Jordan notified the U.S. that the attack's codename was "The Big Wedding" and that it involved airplanes.[106] On August 6, 2001, the CIA's Presidential Daily Brief ("PDB"), designated "For the President Only", was entitled Bin Ladin Determined to Strike in US.

The memo noted that FBI information "indicates patterns of suspicious activity in this country consistent with preparations for hijackings or other types of attacks".[107] In mid-August, one Minnesota flight school alerted the FBI about Zacarias Moussaoui, who had asked "suspicious questions".

The FBI found that Moussaoui was a radical who had traveled to Pakistan, and the INS arrested him for overstaying his French visa.

Their request to search his laptop was denied by FBI headquarters due to the lack of probable cause.[108] The failures in intelligence-sharing were attributed to 1995 Justice Department policies limiting intelligence sharing, combined with CIA and NSA reluctance to reveal "sensitive sources and methods" such as tapped phones.[109] Testifying before the 9/11 Commission in April 2004, then – Attorney General John Ashcroft recalled that the "single greatest structural cause for the September 11th problem was the wall that segregated or separated criminal investigators and intelligence agents".[110] Clarke also wrote: "[T]here were...

failures to get information to the right place at the right time".[111] Early on the morning of Tuesday, September 11, 2001, nineteen hijackers took control of four commercial airliners (two Boeing 757s and two Boeing 767s) en route to California after takeoffs from Logan International Airport in Boston, Massachusetts; Newark Liberty International Airport in Newark, New Jersey; and Washington Dulles International Airport in Loudoun and Fairfax counties in Virginia.[112] Large planes with long coast-to-coast flights were selected for hijacking because they would have more fuel.[113] * Eastern Daylight Time (UTC−04:00)† Excluding hijackers§ Including emergency workers‡ Including hijackers At 7:59 a.m., American Airlines Flight 11 took off from Logan International Airport in Boston.[114] Fifteen minutes into the flight, five hijackers armed with boxcutters took over the plane, injuring at least three people (and possibly killing one)[115][116][117] before forcing their way into the cockpit.

The terrorists also displayed an apparent explosive and sprayed mace into the cabin, in order to frighten the hostages into submission and further hinder resistance.[118] Back at Logan, United Airlines Flight 175 took off at 8:14 a.m., approximately the same time as Flight 11's hijacking.[119] Hundreds of miles southwest at Dulles International Airport, American Airlines Flight 77 left the runway at 8:20 a.m.[119] Flight 175's journey proceeded normally for 28 minutes until 8:42 a.m., when a group of five hijacked the plane, murdering both pilots and stabbing several crew members before assuming control of the aircraft.

These hijackers also used bomb threats to instill fear into the passengers and crew,[120] also spraying chemical weapons to disable any opposition.[121] Concurrently, United Airlines Flight 93 departed from Newark International Airport in New Jersey;[119] originally scheduled to pull away from the gate at 8:00 a.m., the plane was running 42 minutes late.

At 8:46 a.m., Flight 11 was deliberately crashed into the north face of the World Trade Center's North Tower (1 WTC),[122] although the initial presumption by many was that this was merely an accident.[123] At 8:51 a.m., American Airlines Flight 77 was also taken over by another group of five who forcibly entered the cockpit 31 minutes after takeoff.[124] Although the hijackers on this flight were equipped with knives,[125] there were no reports of anyone on board being stabbed, nor did the two people who made phone calls mention the use of mace or a bomb threat.

Seventeen minutes after the first plane crashed into the North Tower, Flight 175 was flown into the South Tower's southern facade (2 WTC)[126] at 9:03 a.m.,[f] demonstrating that the first crash was not an accident, but rather a terrorist attack.[127][128] Four men aboard Flight 93 struck suddenly, killing at least one passenger, after having waited 46 minutes to make their move—a holdup that proved disastrous for the terrorists when combined with the delayed takeoff from the runway;[129] they stormed the cockpit and seized control of the plane at 9:28 a.m., turning the plane eastbound and setting course for Washington, D.C.[130] Much like their counterparts on the first two flights, the fourth team also used bomb threats and filled the cabin with mace.[131] Nine minutes after Flight 93's hijacking, Flight 77 was crashed into the west side of the Pentagon.[132] Because of the two delays,[133] the passengers and crew of Flight 93 had time to be made aware of the previous attacks through phone calls to the ground, and as a result an uprising was hastily organized to take control of the aircraft at 9:57 a.m.[134] Within minutes, passengers had fought their way to the front of the cabin and began breaking down the cockpit door.

Fearing their captives would gain the upper hand, the hijackers rolled the plane and pitched it into a nosedive,[135][136] crashing into a field near Shanksville, Pennsylvania, southeast of Pittsburgh, at 10:03 a.m. The plane was about twenty minutes away from reaching D.C. at the time of the crash, and its target is believed to have been either the Capitol Building or the White House.[113][134] Some passengers and crew members who called from the aircraft using the cabin air phone service and mobile phones provided details: several hijackers were aboard each plane; they used mace, tear gas, or pepper spray to overcome attendants; and some people aboard had been stabbed.[137] Reports indicated hijackers stabbed and killed pilots, flight attendants, and one or more passengers.[112][138] According to the 9/11 Commission's final report, the hijackers had recently purchased multi-function hand tools and assorted Leatherman-type utility knives with locking blades (which were not forbidden to passengers at the time), but these were not found among the possessions left behind by the hijackers.[139][140] A flight attendant on Flight 11, a passenger on Flight 175, and passengers on Flight 93 said the hijackers had bombs, but one of the passengers said he thought the bombs were fake.

The FBI found no traces of explosives at the crash sites, and the 9/11 Commission concluded that the bombs were probably fake.[112] On at least two of the hijacked flights—American 11 and United 93—the terrorists claimed over the PA system that they were taking hostages and were returning to the airport to have a ransom demand met, a clear attempt to prevent passengers from fighting back.

Both attempts failed, however, as both hijacker pilots in these instances (Mohamed Atta[141] and Ziad Jarrah,[142] respectively) keyed the wrong switch and mistakenly transmitted their messages to ATC instead of the people on the plane as intended, tipping off the flight controllers that the planes had been hijacked.

Three buildings in the World Trade Center collapsed due to fire-induced structural failure.

Although the South Tower was struck 17 minutes after the North Tower, the plane's impact zone was far lower, at a much faster speed, and into a corner, with the unevenly-balanced additional structural weight causing it to collapse first at 9:59 a.m.,[144]: 80 [145]: 322 having burned for 56 minutes[h] in the fire caused by the crash of United Airlines Flight 175 and the explosion of its fuel.

The North Tower lasted another 29 minutes before collapsing at 10:28 a.m.,[i] one hour and forty-two minutes[g] after being struck by American Airlines Flight 11.

When the North Tower collapsed, debris fell on the nearby 7 World Trade Center building (7 WTC), damaging the building and starting fires.

These fires burned for nearly seven hours, compromising the building's structural integrity, and 7 WTC collapsed at 5:21 p.m.[149][150] The west side of the Pentagon sustained significant damage.

At 9:42 a.m., the Federal Aviation Administration (FAA) grounded all civilian aircraft within the continental U.S., and civilian aircraft already in flight were told to land immediately.[151] All international civilian aircraft were either turned back or redirected to airports in Canada or Mexico, and were banned from landing on United States territory for three days.[152] The attacks created widespread confusion among news organizations and air traffic controllers.

Among unconfirmed and often contradictory news reports aired throughout the day, one of the most prevalent claimed a car bomb had been detonated at the U.S. State Department's headquarters in Washington, D.C.[153] Another jet (Delta Air Lines Flight 1989) was suspected of having been hijacked, but the aircraft responded to controllers and landed safely in Cleveland, Ohio.[154] In an April 2002 interview, Khalid Sheikh Mohammed and Ramzi bin al-Shibh, who are believed to have organized the attacks, said Flight 93's intended target was the United States Capitol, not the White House.[155] During the planning stage of the attacks, Mohamed Atta (Flight 11's hijacker and pilot) thought the White House might be too tough a target and sought an assessment from Hani Hanjour (who hijacked and piloted Flight 77).[156] Mohammed said al-Qaeda initially planned to target nuclear installations rather than the World Trade Center and the Pentagon, but decided against it, fearing things could "get out of control".[157] Final decisions on targets, according to Mohammed, were left in the hands of the pilots.[156] If any pilot could not reach his intended target, he was to crash the plane.[113] The attack on the World Trade Center's North Tower single-handedly[j] made 9/11 the deadliest act of terrorism in world history.[160] Taken together, the four crashes caused the deaths of 2,996 people (including the hijackers) and injured thousands more.[161] The death toll included 265 on the four planes (from which there were no survivors); 2,606 in the World Trade Center and in the surrounding area; and 125 at The Pentagon.[162][163] Most who died were civilians, as well as 343 firefighters, 72 law enforcement officers, 55 military personnel, and the 19 terrorists.[164][165] After New York, New Jersey lost the most state citizens.[166] More than 90 countries lost citizens in the attacks.[167] In New York City, more than 90% of the workers and visitors who died in the towers had been at or above the points of impact.

In the North Tower, between 1,344[168] and 1,402[169] people were at, above or one floor below the point of impact and all died.

Hundreds were killed instantly the moment the plane struck.[170] The estimated 800 people[171] who survived the impact were trapped and died in the fires or from smoke inhalation; fell or jumped from the tower to escape the smoke and flames; or were killed in the building's collapse.

The destruction of all three staircases in the North Tower when Flight 11 hit made it impossible for anyone from the impact zone upward to escape.

107 people not trapped by the impact died.[172] When the plane struck between floors 93 and 99, the 92nd floor was also rendered inescapable when the crash severed all elevator shafts while debris falling from the impact zone blocked the stairwells, ensuring the deaths of all 69 workers on the floor below the point of impact.

In the South Tower, around 600 people were on or above the 77th floor when Flight 175 struck and few survived.

As with the North Tower, hundreds were killed at the moment of impact.

Unlike those in the North Tower, the estimated 300 survivors[171] of the crash were not technically trapped by the damage done by Flight 175's impact, but most were either unaware that a means of escape still existed or were unable to use it.

One stairway, Stairwell A, narrowly avoided being destroyed as Flight 175 crashed through the building, allowing 14 people located on the floors of impact (including Stanley Praimnath, a man who saw the plane coming at him) and four more from the floors above to escape.

New York City 9-1-1 operators who received calls from people inside the tower were not well informed of the situation as it rapidly unfolded and as a result, told callers not to descend the tower on their own.[173] In total, 630 people died in the South Tower, fewer than half the number killed in the North Tower.[172] Of the 100–200 people witnessed jumping or falling to their deaths that morning,[174] only three recorded sightings were from the South Tower.[175]: 86 Casualties in the South Tower were significantly reduced because some occupants decided to leave the building immediately following the first crash, and because Eric Eisenberg, an executive at AON Insurance, made the decision to evacuate the floors occupied by AON (floors 92 and 98–105) in the moments following the impact of Flight 11.

The 17-minute gap allowed over 900 of the 1,100 AON employees present on-site to evacuate from above the 77th floor before the South Tower was struck; Eisenberg was among the nearly 200 who did not escape.

Similar pre-impact evacuations were carried out by companies such as Fiduciary Trust, CSC, and Euro Brokers, all of whom had offices on floors above the point of impact.