2020 marks the 19th anniversary of the 9/11 terror attack in which nearly 3,000 people were killed when hijackers flew commercial airplanes into New York's World Trade Centre, the Pentagon and a field near Shanksville, Pennsylvania.

Nineteen men affiliated with al-Qaida carried out the four hijackings on September 11, 2001. The tragedy was the deadliest attack on American soil since Pearl Harbor in 1941.

Al-Qaeda and leader Osama bin Laden cited U.S. support of Israel, the presence of U.S. troops in Saudi Arabia, and sanctions against Iraq as motives behind the attacks.

The United States searched for bin Laden for years after the terrorist attacks, and in May 2011 a team of U.S. Navy SEALs raided a compound in Abbottabod, Pakistan, where they shot him dead.

Memorials for those killed in the attacks now stand at all three sites in New York, Virginia and Pennsylvania.