By Jonathan Allen
NEW YORK (Reuters) - 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 U.S. Defense 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, is now increasingly reconnected with the surrounding streets as rebuilding at the site nears completion.
"For the first time this year, because the museum opened in May, family members will be able to visit the museum as part of the commemoration," said Michael Frazier, a memorial spokesman.
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 still weighs.
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 outlined plans to attack 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 general public is at the Flight 93 National Memorial in Shanksville, Pennsylvania, which marks the field where one of the four hijacked airliners crashed.
The Congressional Gold Medal honoring the flight's passengers and crew will go on public display for the first time, the National Park Service said.
(Editing by Mohammad Zargham)