The September 11 attacks were carried out by terrorists.
Al-Qaeda ( [ ælˈkaɪdə ] or [ ˌælkɑːˈiːdə ] القاعدة , [ ælqɑːʕɪdɐ ] , translation : `` The Base '' , `` The Foundation '' or `` The Fundament '' and alternatively spelled al-Qaida , al-Qæda and sometimes al-Qa ` ida ) is a militant Sunni Islamist multi-national organization founded in 1988 by Osama bin Laden , Abdullah Azzam , and several other Arab volunteers who fought against the Soviet invasion of Afghanistan in the 1980s . 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 . 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 .