On September 11th 2001, terrorists hijacked four commercial airliners flying over the eastern United States.
Two planes crashed into the towers of the World Trade Centre, the third into The Pentagon, and the fourth crashed in a field in Pennsylvania after passengers fought with the hijackers to try and regain control of the plane.
Nearly 3000 people (246 passengers aboard the aircraft, 2606 in the World Trade Centre and 125 people in the Pentagon) died in this event.
Of those killed 415 were emergency workers, 343 being firefighters.
Today we remember those fallen, their families, colleagues and friends.