Until a few months ago, the part of New York City where crowds will gather on Thursday morning to mark the 13th anniversary of the Sept. 11 attacks on the United States had been mostly fenced off to the public.

This year, for perhaps the first time since the attacks, a sense of normalcy and openness has taken root in the city blocks where two airliners hijacked by militants from al Qaeda crashed into the World Trade Center's twin towers.

Rebuilding efforts at the site, where 2,753 people died, are nearing completion. 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.

Against that backdrop, politicians, families of those who died in the attacks and other dignitaries will gather on Thursday to observe moments of silence and hear recitations of nearly 3,000 victims' names. It has become an annual ritual.

Similar ceremonies will also be held in Washington, where a hijacked plane plowed into the Pentagon, and the field in Shanksville, Pennsylvania, where another hijacked plane crashed.

In New York, it is the first commemoration ceremony since the opening of the 9/11 museum and the adjoining repository for unidentified human remains at the site. That is an important milestone for families of the victims, officials say.

"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," Michael Frazier, a museum spokesman, said.

While lower Manhattan may look and feel different this year, the external threat to the United States represented by the 9/11 attacks still weighs. Islamic State, a militant group that began as an offshoot of al Qaeda, is viewed by Washington as an increasing danger.

The Islamist group, which released videos of its fighters beheading two American hostages, has even revived fears of an attack on American soil.

U.S. President Barack Obama is expected to announce his strategy to fight Islamic State on Wednesday evening, about 12 hours before the Sept. 11 commemorations begin.

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.

Thousands of tourists pose for photographs each day around the two memorial waterfalls that mark the footprints of the towers, set in a paved plaza dotted with trees, before lining up to visit the subterranean museum about the attack.

Critics have said that the plaza, with its unusually prohibitive rules for a city public space and deliberate lack of garbage cans, is of little use to people who live or work in the area.

Michael Kimmelman, architecture critic for the New York Times, described the plaza as "formal, gigantic, impersonal, flat, built to awe, something for tourists."

Obama is expected to speak at the Pentagon, the headquarters of the United States Department of Defense, during a private ceremony on Thursday morning for relatives of the people killed in the attack on the building.

The only ceremony open to the general public is at the Flight 93 National Memorial in Pennsylvania, which marks the site where one of the four airliners crashed.

The Congressional Gold Medal, the nation's highest civilian award, honoring the passengers and crew who were aboard that flight, will go on public display for the first time, the National Park Service said.

(This story corrects paragraph 11 to show 1 World Trade Center due to open later this year, not next year)