After viewing some of the exhibits, including a mangled fire truck and a memorial wall with photos of victims, the president touched on some of the many stories of courage amid the chaos: the passengers who stormed a hijacked plane's cockpit over a Pennsylvania field and first responders who rushed into the burning twin 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 stricken south tower. He died in the tower's collapse.
One of the red bandannas he made a habit of carrying is in the museum, and Crowther's 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. This is the true legacy of Sept. 11."
By her side was Ling Young, one of the people Welles Crowther rescued.
"It was very hard for me to come here today," but she wanted to thank his parents, she said.
Before the ceremony, Obama walked quietly through an expansive hall with former New York City Mayor Michael Bloomberg. First lady Michelle Obama, former President Bill Clinton and former Secretary of State Hillary Rodham Clinton following behind them.
The museum, which commemorates the 2001 terrorist attack, as well as the 1993 World Trade Center bombing, opens to the public on May 21.
Reflections from dignitaries — including New York Gov. Andrew Cuomo, New Jersey Gov. Chris Christie, current New York Mayor Bill de Blasio and former Mayor Rudy Giuliani — 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 and tax dollars.