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 . It has been designated as a terrorist group by the United Nations Security Council , the North Atlantic Treaty Organization ( NATO ) , the European Union , the United States , Russia , India , and various other countries ( see below ) . September 11 attacks . 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 .