Claim: The September 11 attacks occurred in the twenty-first century.

The September 11 attacks -LRB- also referred to as 9/11 -RRB- The expression `` 9/11 '' is pronounced `` nine eleven '' . The slash is not part of the pronunciation . The name is frequently used in British English as well as in American English , even though the dating conventions differ . were a series of four coordinated terrorist attacks by the Islamic terrorist group al-Qaeda on the United States on the morning of Tuesday , September 11 , 2001 . The attacks killed 2,996 people , injured over 6,000 others , and caused at least $ 10 billion in property and infrastructure damage .   Four passenger airliners operated by two major U.S. passenger air carriers -LRB- United Airlines and American Airlines -RRB- -- all of which departed from airports in the northeastern United States bound for California -- were hijacked by 19 al-Qaeda terrorists . Two of the planes , American Airlines Flight 11 and United Airlines Flight 175 , were crashed into the North and South towers , respectively , of the World Trade Center complex in New York City . Within an hour and 42 minutes , both 110-story towers collapsed , with debris and the resulting fires causing partial or complete collapse of all other buildings in the World Trade Center complex , including the 47-story 7 World Trade Center tower , as well as significant damage to ten other large surrounding structures . A third plane , American Airlines Flight 77 , was crashed into the Pentagon -LRB- the headquarters of the United States Department of Defense -RRB- in Arlington County , Virginia , leading to a partial collapse of the building 's western side . The fourth plane , United Airlines Flight 93 , initially was steered toward Washington , D.C. , but crashed into a field in Stonycreek Township near Shanksville , Pennsylvania , after its passengers tried to overcome the hijackers . It was the deadliest incident for firefighters and law enforcement officers in the history of the United States , with 343 and 72 killed respectively .   Suspicion for the attack quickly fell on al-Qaeda . The United States responded to the attacks by launching the War on Terror and invading Afghanistan to depose the Taliban , which had harbored al-Qaeda . Many countries strengthened their anti-terrorism legislation and expanded the powers of law enforcement and intelligence agencies to prevent terrorist attacks . Although al-Qaeda 's leader , Osama bin Laden , initially denied any involvement , in 2004 he claimed responsibility for the attacks . Al-Qaeda and bin Laden cited U.S. support of Israel , the presence of U.S. troops in Saudi Arabia , and sanctions against Iraq as motives . Having evaded capture for almost a decade , bin Laden was located and killed by SEAL Team Six of the U.S. Navy in May 2011 .   The destruction of the World Trade Center and nearby infrastructure caused serious damage to the economy of Lower Manhattan and had a significant effect on global markets , closing Wall Street until September 17 and the civilian airspace in the U.S. and Canada until September 13 . Many closings , evacuations , and cancellations followed , out of respect or fear of further attacks . Cleanup of the World Trade Center site was completed in May 2002 , and the Pentagon was repaired within a year . On November 18 , 2006 , construction of One World Trade Center began at the World Trade Center site . The building was officially opened on November 3 , 2014 . Numerous memorials have been constructed , including the National September 11 Memorial & Museum in New York City , the Pentagon Memorial in Arlington County , Virginia , and the Flight 93 National Memorial in a field in Stonycreek Township near Shanksville , Pennsylvania .