sentences:  The September 11 attacks – which were described as a "watershed moment" of contemporary history – were a series of coordinated suicide attacks by Al-Qaeda upon the United States on 11 September 2001.On that morning, 19 Al-Qaeda terrorists hijacked four commercial passenger jet airliners.The hijackers intentionally crashed two of the airliners into the Twin Towers of the World Trade Center in New York City, killing everyone on board and many others working in the buildings.