President Barack Obama has joined September 11 survivors and rescuers at the dedication of a memorial museum on the site of the attacks in New York.
Mr Obama told those gathered it was a “sacred place of healing and of hope”.
The National September 11 Memorial Museum includes thousands of personal items and parts of the World Trade Center towers themselves.
Almost 3,000 people died on 11 September 2001 after al-Qaeda hijackers flew aeroplanes into the towers.
Another hijacked plane hit the Pentagon. A fourth crashed into a field in Pennsylvania after passengers fought with the hijacker.
In his opening remarks at the ceremony, former New York Mayor Michael Bloomberg said the museum was “a reminder to us and all future generations that freedom carries heavy responsibilities”.
President Obama said the museum means we can all “look into the faces of nearly 3,000 innocent souls”.
“We can touch their names and hear their voices, glimpse the small items that speak to the beauty of their lives – a wedding ring, a dusty helmet, a shining badge,” he told those gathered.
As well as rescuers, survivors and relatives of people who lost their lives, there was in attendance the New York mayor at the time of the attacks, Rudy Giuliani, the present mayor, Bill de Blasio, and actor Robert De Niro.
Many more watched a broadcast of the ceremony from the plaza outside the museum.
In his short speech, the president recalled the story of Welles Crowther, a 24-year-old World Trade Center worker and former volunteer firefighter who became known as “the man in the red bandana” after leading workers to safety before dying in the south tower’s collapse.
His bandana is in the museum and his mother, Alison, told the audience she hoped it would remind visitors “how people helped each other that day, and that they will be inspired to do the same in ways both big and small”.
The museum features dramatic and horrific moments of the day in videos, including the two skyscrapers collapsing, but also symbols of heroism, such as damaged fire trucks and the wristwatch of one of the passengers who confronted the hijackers.
Before the ceremony, President Obama and First Lady Michelle Obama toured the museum, viewing a memorial wall with photos of victims and a mangled fire truck.
Along with the nearby memorial plaza, the New York city museum cost $700m (£418m) in donations and public money.
The museum, not far from the original site of the World Trade Center, is largely underground. It will be fully open to the public on 21 May.
The museum is not without controversy. Some relatives of victims are upset that unidentified humans remains found in the rubble will be located near the museum at Ground Zero.
Some Muslim groups have also said a video describing al-Qaeda and the run-up to the attacks does not differentiate enough between the violent hijackers motivated by a radical vision of Islam and regular Muslims.