Americans are marking the 19th anniversary of the September 11, 2001, terrorist attacks that killed almost 3,000 people in New York, Virginia and Pennsylvania.
What happened that day?
Nineteen men affiliated with al-Qaida carried out the four hijackings.
The deadliest attack on American soil since Pearl Harbor in 1941, the events of September 11 permanently changed America's perception of security and prompted then-President George W. Bush to declare war on terrorism and invade Afghanistan.
US war on terror
U.S.-led coalition forces knocked the Taliban from power in Afghanistan. The war there continues, although earlier this year the U.S. and Taliban signed an agreement aimed at ending the conflict.
The United States searched for al-Qaida leader Osama 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 honoring the victims
Memorials for those killed in the attacks now stand at all three sites in New York, Virginia and Pennsylvania.
The Pentagon crash site was quickly rebuilt after the attacks.
A new tower at the World Trade Center site took longer to construct, but now rises above the Manhattan skyline as the tallest building in the United States.