The 9/11 Memorial and Museum located at Ground Zero in New York City. The 9/11 Memorial Museum serves as America's principal institution concerned with exploring the implications of the events of 9/11, documenting the impact of those events and exploring 9/11's continuing significance. The September 11, 2001 attacks, also called 9/11 attacks, were a series of airline hijackings and suicide attacks committed in 2001 by 19 militants associated with the Islamic extremist group al-Qaeda against targets in the United States. The deadliest terrorist attacks on American soil in U.S. history. The attacks against New York City and Washington, D.C., caused extensive death and destruction and triggered an enormous U.S. effort to combat terrorism. Some 2,750 people were killed in New York, 184 at the Pentagon, and 40 in Pennsylvania (where one of the hijacked planes crashed after the passengers attempted to retake the plane); all 19 terrorists died. Police and fire departments in New York were especially hard-hit: hundreds had rushed to the scene of the attacks, and more than 400 police officers and firefighters were killed.