NEW YORK - Politicians, dignitaries and victims' relatives were gathering in New York, Washington and Pennsylvania on Thursday to commemorate the nearly 3,000 people killed in al Qaeda's attack on the United States 13 years ago on Sept. 11.
In what has become an annual ritual, the names of the victims will be read aloud at a ceremony in lower Manhattan, punctuated by moments of silence to mark the times when each of the four hijacked airliners crashed and the World Trade Center's twin towers fell.
President Barack Obama is due to speak at the Pentagon during a private ceremony for relatives of the people killed in the attack on the headquarters of the US Defence Department by the Islamist militant group.
In New York City, it is the first commemoration ceremony since the opening of the museum at the National September 11 Memorial, along with the adjoining repository for unidentified victims' remains.
The area, by turns a smoldering grave and an off-limits construction site for more than a decade, now is increasingly reconnected with the surrounding streets as rebuilding at the site nears completion.
Larry and Rachel Meltzer arrived an hour before the ceremony's start, carrying foldable chairs and wearing badges that bore a picture of Larry's brother, Stuart, who was a trader in one of the towers when he was killed.
"It's hard to believe it's been 13 years," Meltzer said before heading through the security checkpoint. "As far as the pain, I never forget and it never diminishes."
Although the reconstruction has been plagued by delays, two of the new skyscrapers built around the site of the fallen twin towers are now open, while 1 World Trade Center, the tallest skyscraper in the Western Hemisphere, is due to open later this year.
While lower Manhattan may look and feel different this year, the external threat to the United States represented by the Sept. 11 attacks remains.
The United States and its allies see Islamic State, a group that began as an offshoot of al Qaeda, as an increasing danger. On Wednesday, Obama said he had ordered an aerial bombing campaign on the group, which has seized large parts of Iraq and Syria and released videos of beheadings of two American hostages.
The only ceremony open to the public is at the Flight 93 National Memorial in Shanksville, Pennsylvania, where one of the four hijacked airliners crashed.