Al-Qaeda executed the September 11 attacks.
Two weeks after the September 11 attacks , the Federal Bureau of Investigation connected the hijackers to al-Qaeda , a global , decentralized terrorist network . September 11 attacks . 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 . Four passenger airliners operated by two major U.S. passenger air carriers ( United Airlines and American Airlines ) -- all of which departed from airports in the northeastern United States bound for California -- were hijacked by 19 al-Qaeda terrorists . Although al-Qaeda 's leader , Osama bin Laden , initially denied any involvement , in 2004 he claimed responsibility for the attacks . United Airlines Flight 93 was a domestic scheduled passenger flight that was hijacked by four Al-Qaeda terrorists on board , as part of the September 11 attacks .