Obama attends 9/11 museum ceremony
Mr Obama walked quietly through an expansive hall with former New York mayor Michael Bloomberg, with the first lady, former president Bill Clinton and former secretary of state Hillary Clinton following behind.
Mr Obama was also speaking at the dedication ceremony, with hundreds of dignitaries and relatives of September 11 victims, as well as survivors and rescuers, there to view the museum built to commemorate the 2001 terrorist attack, as well as the 1993 World Trade Centre bombing.
The museum opens to the public on May 21.
By turns chilling and heartbreaking, the ground zero museum leads people on an unsettling journey through the terrorist attacks, with forays into their lead up and legacy.
There are scenes of horror, including videos of the skyscrapers collapsing and people falling from them. But there also are symbols of heroism, ranging from damaged fire trucks to the watch of one of the airline passengers who confronted the hijackers.
"You won't walk out of this museum without a feeling that you understand humanity in a deeper way," said museum president Joe Daniels.
The museum and memorial plaza above, which opened in 2011, were built for 700 million US dollars (£416 million) in donations and tax dollars.
Work on the museum was marked by construction problems, financial squabbles and disputes over its content and the appropriate way to honour the dead, but its leaders see it as a monument to unity and resilience.
And its opening is prompting reflection from presidents and the everyday people whose lives were changed by the attacks.
Former president George W Bush issued a statement saying the museum "will help ensure that our nation remembers the lessons of September 11".
Visitors start in an airy pavilion where the rusted tops of two of the World Trade Centre's trident-shaped columns shoot upwards.
From there, museum-goers descend stairs and ramps, passing through a dark corridor filled with the voices of people remembering the day and past the battered "survivors' staircase" that hundreds used to escape the burning towers.
At the base level - 70ft below ground, amid remnants of the skyscrapers' foundations - there are such artefacts as a mangled piece of the antenna from the top the trade centre and a fire truck with its cab shorn off.
Then, galleries plunge visitors into the chaos of September 11: fragments of planes, a set of keys to the trade centre, a teddy bear left at the impromptu memorials that arose after the attacks, the dust-covered shoes of those who fled the skyscrapers' collapse, emergency radio transmissions and office workers calling loved ones, even a recording of an astronaut solemnly describing the smoke plume from the International Space Station.
Sprinkled in are snippets about the 19 hijackers, including photos of them on an inconspicuous panel.
Speaking at the ceremony, Mr Obama said no act of terror can match the strength and character of the United States and the true spirit of that terrible day is love, compassion and sacrifice.
He said "nothing can ever break us", adding that it is a moment to reflect on the true spirit of 9/11 and enshrine it forever in the nation's heart.
He praised the men and women who helped save lives in the attack, including those who gave their lives in the effort.
He said the deceased live on in us and in their friends and families.
The president praised the new museum as "a sacred place of healing and of hope" that captures both the story and the spirit of heroism and helping others that followed the attacks.
"It's an honour to join in your memories, to recall and to reflect, but above all to reaffirm the true spirit of 9/11 - love, compassion, sacrifice - and to enshrine it forever in the heart of our nation," he said.
"Like the great wall and bedrock that embrace us today, nothing can ever break us. Nothing can change who we are as Americans."
Meanwhile, Mr Bloomberg said: "Walking through this museum can be difficult at times, but it is impossible to leave without feeling inspired."