The September 11 attacks, also known as the 9/11 attacks, were a series of airline hijackings and suicide attacks carried out in 2001 against targets in the United States by 19 militants affiliated with the Islamic extremist group al-Qaeda, and were the deadliest terrorist attacks on American soil in US history. The attacks on New York City and Washington, D.C., caused widespread death and destruction and sparked a massive counter-terrorism effort in the United States. In New York, 2,750 people were killed, 184 at the Pentagon, and 40 in Pennsylvania (where one of the hijacked planes crashed after passengers attempted to retake the plane); all 19 terrorists were killed.