The September 11 attacks were carried out by 19 members of al-Qaeda.
The September 11 , 2001 , attacks in the northeast United States were an organized terrorist act carried out by 19 hijackers , and organized by numerous members of al-Qaeda . The September 11 attacks ( also referred to as 9/11 ) The expression `` 9/11 '' is pronounced `` nine eleven '' . 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 .