On September 11, 2001, a series of four coordinated terrorist attacks were carried out by al-Qaeda on the United States.
Two aircraft –American Airlines flight 11 and United Airlines flight 175 - were flown into the towers of the World Trade Center in New York City, a third plane – American Airlines flight 77 - hit the Pentagon near Washington DC and a fourth – United Airlines flight 93 - crashed in a field in Pennsylvania.
More than 3000 people were killed during the attacks, including more than 400 police and fire fighters.
An anniversary ceremony at the 9/11 Memorial Plaza will include six moments of silence; to mark when the WTC towers were struck, when they fell, when the Pentagon was attacked and when flight 93 crashed in Pennsylvania.
Where were you on September 11, 2001?