The United States is marking the 25th anniversary of the 9/11 attacks with memorial ceremonies in New York, Pennsylvania and Virginia
Speaking earlier in a ceremony to honour the 184 people killed when American Airlines Flight 77 crashed into the Pentagon, President Trump said "we will never, ever forget"
The other living US presidents - Bill Clinton, George W Bush, Barack Obama and Joe Biden - all attended the ceremony in New York at the site of the World Trade Center where about 2,700 died after hijackers crashed two planes into the Twin Towers
Another plane crashed in a field in Pennsylvania - a memorial service is taking place there to remember those who died on board Flight 93 after passengers tried to regain control of the cockpit
"Everybody helped everybody," a New Yorker tells the BBC as he says the togetherness that emerged from the attacks was the "best part that came out of that terrible day"
A day that "changed the landscape of the country" and "something that we all should remember".
Those are some of the words Americans have chosen to describe the 9/11 terror attacks 25 years on.
One man recalls waking up that day, "getting a phone call, turning on the TV and just crying".
Another woman describes the "heartbreaking" moment she saw footage of the towers falling.
Across America, people have already been paying tribute to the victims of 9/11.
It's been 25 years since the attacks and commemorative events and displays have lit up the sky above New York and nearly 3,000 US flags have been laid in the shape of the Twin Towers in North Carolina.
Here are some of the pictures we've seen so far:
Image source, Getty ImagesThe Tribute in Light public art installation commemorating the September 11 attacks lights up the sky above New York
Image source, Getty ImagesIn Charlotte, North Carolina, people have laid nearly 3,000 US flags to remember the 9/11 victims
Image source, ReutersPeople take part in a 9/11 tribute at the Republican National Midterm Convention in Dallas, Texas
Frank Gardner
Security correspondent
Image source, Getty ImagesThe first duty of a government, countless leaders over the decades have said, is to safeguard its population. On 11 September 2001 the US manifestly failed to do that. The intelligence was there, yet tragically, nobody joined up the dots in time.
9/11, as the attacks on the US became known, came to define – along with the response – a generation of foreign, security and intelligence policy for the first two decades of this century.
In the ensuing 25 years there have been numerous intelligence successes as well as failures on both sides of the Atlantic. The successes include plots that have been foiled, suspects who have been arrested, brought to trial, convicted and jailed.
But what of the bigger picture? How has intelligence-gathering, counter-terrorism and strategic planning changed in the 25 years since the worst terrorist attack in US history?
Image source, ReutersIn all, 2,977 people (not counting the 19 hijackers) lost their lives in the September 11 attacks, most of them in New York City:
The youngest victim was two-year-old Christine Lee Hanson and the oldest was 82-year-old Robert Norton, who were both passengers on planes.
Citizens of more than 90 different countries were among the casualties. New York City lost 441 first responders - the largest loss of emergency personnel in US history.
Thousands of people were injured, or later developed illnesses connected to the attacks, including firefighters who had worked in toxic debris.
Image source, Getty ImagesThe Wall of Names memorial honouring the 40 passengers and crew on Flight 93
United Flight 93 was the only plane that did not hit any targets during the attacks of 9/11.
The flight from Newark, New Jersey was headed west to San Francisco when four hijackers gained control of the aircraft and changed its flight path to travel south, heading towards the Washington DC area.
It is thought the hijackers had meant to attack Washington's Capitol Building - the attacks occurred days after US lawmakers resumed legislative activities after their traditional summer break.
The 40 passengers and crew onboard Flight 93 learned about the attacks in New York and at the Pentagon by communicating with friends and family on airfones.
They fought back against the hijackers who crashed the plane in an open field in Shanksville, Pennsylvania killing all on board and saving hundreds of lives.
A memorial, known as the Wall of Names, stands in the field where the plane crashed to honour the victims and the heroism they showed that day.
Image source, Getty ImagesOn Tuesday 11 September 2001, four planes flying over the eastern US were seized by small teams of hijackers from the Islamist extremist network al-Qaeda.
They were then used as giant, guided missiles to crash into landmark buildings in New York and Virginia. Two planes hit the Twin Towers of the World Trade Center in New York, with the second strike occurring 17 minutes after the first.
The buildings caught fire, trapping people on the upper floors, and enveloping the city in smoke. In less than two hours, both 110-storey towers collapsed in massive clouds of dust.
A third plane destroyed the western face of the Pentagon - the US military headquarters located just outside 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.
In all, 2,977 people (not counting the 19 hijackers) lost their lives, most of them in New York, including 441 first responders - the largest loss of emergency personnel in US history.
Image source, Spencer Platt/Getty ImagesPeople pictured attending a memorial service at 9/11 Memorial & Museum on September 11 last year
A series of memorials and vigils are being held across the US today, with the main events being held at the three sites where the attacks took place. Here's what to expect throughout the day:
In New York
In Pennsylvania
In Virginia
Image source, Getty ImagesToday marks 25 years since the 9/11 attacks, as the world comes together to remember the 2,977 people who lost their lives.
On Tuesday 11 September 2001, suicide attackers hijacked four US passenger jets and crashed them into the Twin Towers of the World Trade Center in New York, which subsequently collapsed, and the Pentagon in Virginia.
The fourth plane crashed in a field outside Shanksville, Pennsylvania, after passengers fought back against the hijackers.
Twenty five years on from the attacks, which claimed the lives of people from more than 90 countries, their impact continues to be felt around the world.
A commemoration event is due to be held at 11:00 BST at the 9/11 memorial sculpture in London's Queen Elizabeth Olympic Park.
In the US, memorial events are being held throughout the day to honour those who lost their lives, including at the sites of the three attacks in New York City, at the Pentagon and in Pennsylvania.
The names of all 2,977 victims will be read out by relatives during a special ceremony at the World Trade Center at 08:30 EST (13:30 BST), which will be punctuated by seven moments of silence.
We'll bring you the latest updates throughout the day on how the anniversary is being marked in the US and around the world.