On Tuesday 11 September 2001, hijackers seized US passenger jets and crashed them into two New York skyscrapers, killing thousands of people.
A third plane destroyed the western face of the Pentagon - the giant headquarters of the US military just outside the nation's capital, Washington DC.
The fourth plane crashed in a field in Pennsylvania after passengers fought back. It is thought the hijackers had meant to attack Washington's Capitol Building.
The BBC spoke to Americans in Washington about what they were doing on that day and how 9/11 has changed the country.
Video by Meiying Wu
