US President Barack Obama (2nd L), former New York Mayor Michael Bloomberg (L), first lady Michelle Obama (2nd R), former US President Bill Clinton (R) and his wife, former Secretary of State Hillary Clinton, look at the faces of those who died during the 9/11 attacks, at the National September 11 Memorial Museum in New York, May 15, 2014. [Photo/Agencies]

NEW YORK - US President Barack Obama praised the new Sept 11 museum on Thursday as "a sacred place of healing and of hope" that captures both the story and the spirit of heroism that followed the attacks.

"Like the great wall and bedrock that embrace us today, nothing can ever break us. Nothing can change who we are as Americans," he told an audience of victims' relatives, survivors and rescuers at the ground zero museum's dedication ceremony.

The museum commemorates Sept 11, 2001, when 19 al-Qaida hijackers crashed four airliners into the World Trade Center, the Pentagon and a field in Pennsylvania. Nearly 3,000 people were killed in an attack that plunged the US into a decade of war in Afghanistan against al-Qaida's Taliban protectors.

After viewing some of the exhibits, including a mangled fire truck and a memorial wall with photos of victims, Obama touched on stories of courage amid the chaos 13 years ago: the passengers who stormed a hijacked plane's cockpit and first responders who rushed into the burning World Trade Center towers. He also honored military members "who have served with honor in more than a decade of war."

He focused especially on 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 bandanna" after he led other workers to safety from the trade center's south tower. He died in the tower's collapse.

Ling Young, one of the people Crowther rescued, said "it was very hard for me to come here today," but she wanted to thank his parents.

Before the ceremony, Obama walked quietly through an expansive hall with former New York City Mayor Michael Bloomberg. First lady Michelle Obama, former US President Bill Clinton and former US Secretary of State Hillary Rodham Clinton followed.

Notably absent was former US President George W. Bush, whose presidency was defined by the attacks less than a year into his first term.

Bush was invited, according to the museum. But Bush spokesman Freddy Ford said he was unable to attend because of a scheduling conflict.

Reflections from dignitaries at Thursday's ceremony were interspersed with the voices of everyday people caught up in Sept 11.

Retired Fire Department Lt. Mickey Cross described being trapped for hours in the wreckage of the north tower - and then joining the recovery effort after being rescued. "There was a real sense of caring for each other," he said.

Ada Dolch, a school principal whose sister died at the trade center, recalled turning her grief into inspiration to open a school in Afghanistan. "What a kick in the head to Osama bin Laden!" she said.

Kayla Bergeron remembered walking down 68 flights of stairs in the north tower, amid confusion and fear that there was no way out. Her final steps to safety were on an outdoor stairway, now in the museum as the "survivors' stairs."

"Today, when I think about those stairs, what they represent to me is resiliency," she said.

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 wristwatch of one of the airline passengers who confronted the hijackers.

The museum and memorial plaza above, which opened in 2011, were built for $700 million in donations. It opens to the public on May 21.

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