US President Barack Obama has hailed the love and sacrifice he said was "the true spirit of 9/11" as he inaugurated a Ground Zero museum about Al-Qaeda attacks which killed nearly 3,000 people.
President Obama said the museum, in the footprint of the former World Trade Centre Twin Towers, would ensure that the horror and heroism of 11 September, 2001 would never be forgotten by future generations.
He said it was an honour to recall "the true spirit of 9/11, love, compassion, sacrifice and to enshrine it forever in the heart of our nation."
"I want to express our deep gratitude to everybody who was involved in this great undertaking for bringing us to this day, for giving us this sacred place of healing and of hope," he said.
Former New York mayor Michael Bloomberg said the museum was a "sacred marker of our past" and a "solemn gathering place."
The museum, with its 10,000 artifacts, provides an emotionally bracing experience likely to revive terrible memories of the attacks.
Inside, a map charts the path of four hijacked airliners sent on a suicide mission on a crisp but fateful morning nearly 13 years ago.
There are poignant reminders of the people who went to work that day and never came home, a set of charred credit cards and a woman's shoe, as well as a crushed and burned fire engine memorializing the 343 New York firefighters who died.
A huge iron column, the last recovered from the site in May 2002, is on display, along with stairs from a nearby street that were used by hundreds of people to flee the site of the drama.
In the bowels of the museum, visitors can examine the foundations of one of the towers which once defined New York's skyline but which collapsed in a murderous cloud of fire and ash after being hit by planes used as massive suicide bombs.
Exhibits take an in-depth look at the events of 11 September, including an analysis of the background to the attacks and an examination of how the after-effects are still being felt today.
Visitors, who are barred from taking photographs, can hear the last telephone messages to loved ones left by New Yorkers trapped in the burning towers.
In all, there are 23,000 still images, more than 500 hours of film and video and more than 2,000 archival documents at the museum.