On Tuesday 11 September 2001 suicide attackers seized US passenger jets and crashed them into two New York skyscrapers, killing thousands of people.
The attack remains one of the most traumatic events of the century, not only for Americans but also for the world.
What were the targets?
Four planes flying over the eastern US were seized simultaneously by small teams of hijackers.
They were then used as giant, guided missiles to crash into landmark buildings in New York and Washington.
Two planes struck the Twin Towers of the World Trade Center in New York.
The first hit the North Tower at 08:46 Eastern Time (13:46 GMT). The second crashed into the South Tower at 09:03.
The buildings were set on fire, trapping people on the upper floors, and wreathing the city in smoke. In less than two hours, both 110-storey towers collapsed in massive clouds of dust.
At 09:37 the 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 at 10:03 after passengers fought back. It is thought the hijackers had meant to attack the Capitol Building in Washington DC.
How many people died?
In all, 2,977 people (not counting the 19 hijackers) lost their lives, most of them in New York.
- All 246 passengers and crew aboard the four planes were killed
- At the Twin Towers, 2,606 people died – then or later of injuries
- At the Pentagon, 125 people were killed
The youngest victim was two-year-old Christine Lee Hanson, who died on one of the planes with her parents Peter and Sue.
The oldest was 82-year-old Robert Norton, who was on another plane with his wife Jacqueline, en route to a wedding.
When the first plane struck, an estimated 17,400 people were in the towers. Nobody survived above the impact zone in the North Tower, but 18 managed to escape from the floors above the impact zone in the South Tower.
Citizens of 77 different countries were among the casualties. New York City lost 441 first responders.
Thousands of people were injured or later developed illnesses connected to the attacks, including firefighters who had worked in toxic debris.
Who were the attackers?
An Islamist extremist network called al-Qaeda planned the attacks from Afghanistan.
Led by Osama Bin Laden, al-Qaeda blamed the US and its allies for conflicts in the Muslim world.
Nineteen people carried out the hijackings, working in three teams of five and one of four (on the plane which crashed in Pennsylvania).
Each group included someone who had received pilot training. This was carried out at flying schools in the US itself.
Fifteen hijackers were Saudis like Bin Laden himself. Two were from the United Arab Emirates, one was from Egypt and one was from Lebanon.
How did the US respond?
Less than a month after the attacks, President George W Bush led an invasion of Afghanistan – supported by an international coalition – to eradicate al-Qaeda and hunt down Bin Laden.
However, it was not until 2011 that US troops finally located and killed Bin Laden in neighbouring Pakistan.
The alleged planner of the 9/11 attacks, Khalid Sheikh Mohammad, was arrested in Pakistan in 2003. He has been held in US custody at Guantanamo Bay since then, and is still awaiting trial.
Al-Qaeda still exists. It is strongest in Sub-Saharan Africa but even now has members inside Afghanistan.
US troops left Afghanistan this year after nearly 20 years, stoking fears from many that the Islamist network could make a comeback.
The legacy of 9/11
Flight safety was tightened around the world in the years following 9/11.
In the US, the Transportation Security Administration was created to beef up security at airports and on planes.
It took more than eight months to clean up “Ground Zero” – the site of the fallen Twin Towers.
A memorial and a museum now stands on the site, and buildings have risen up again, to a different design.
The completed centrepiece – One World Trade Center, or “Freedom Tower” – stands even higher (1,776ft (541m) than the original North Tower, which was 1,368ft.
Reconstruction at the Pentagon took just under a year, with staff back in their offices by August 2002.