NEW YORK – The U.S. marked the 15th anniversary of 9/11 on Sunday, with victims’ relatives reading their names and reflecting on a loss that still felt as immediate to them as it was indelible for the nation.

“It doesn’t get easier. The grief never goes away. You don’t move forward – it always stays with you,” Tom Acquaviva, who lost his son, Paul Acquaviva, said as he joined over 1,000 victims’ family members, survivors and dignitaries at ground zero under an overcast sky.

For Dorothy Esposito, too, the 15 years since she lost her son, Frankie, is “like 15 seconds.”

James Johnson, a retired New York City police sergeant who is now police chief in Forest City, Pennsylvania, was there for the first time since he last worked on the rescue and recovery efforts in early 2002.

“I’ve got mixed emotions, but I’m still kind of numb,” he said. “I think everyone needs closure, and this is my time to have closure.”

President Obama was scheduled to speak at an observance at the Pentagon. Hundreds of people also were expected at a ceremony at the Flight 93 National Memorial in Shanksville, Pennsylvania.

Nearly 3,000 people died when hijacked planes slammed into the World Trade Center, the Pentagon and a field near Shanksville on Sept. 11, 2001. It was the deadliest terror attack on American soil.

The 15th anniversary arrives in a country caught up in a combustible political campaign and keenly focused on political, economic and social fissures.

But some at the ceremony pleaded for the nation to look past its differences.

“The things we think separate us really don’t. We’re all part of this one Earth in this vast universe,” said Granvilette Kestenbaum, who lost her astrophysicist husband, Howard Kestenbaum. “We’re all ordinary, and we’re all special, we’re all connected. We waste precious time by thinking otherwise.”

Others expressed hopes for peace or alluded to the presidential race, calling on the next commander-in-chief to ensure America’s safety.

Still, the nation tries to put partisan politics on hold on the anniversary, and both Democratic presidential candidate Hillary Clinton and Republican rival Donald Trump were at the anniversary ceremony at the World Trade Center. Neither candidate made public remarks at a ceremony where politicians have been allowed to attend, but not speak, since 2011.

Clinton’s campaign, however, says she left the ceremony early after feeling “overheated.”

Clinton spokesman Nick Merrill said in a statement Sunday that the former secretary of state attended the morning ceremony for 90 minutes “to pay her respects and greet some of the families of the fallen.

He added, “During the ceremony, she felt overheated so (she) departed to go to her daughter’s apartment, and is feeling much better.”

The statement offered no additional details, including whether Clinton required medical attention.

Clinton and Trump also followed a custom of halting television ads for the day.

Ceremony organizers included some additional music and readings Sunday to mark the milestone year. But they kept close to what are now traditions: moments of silence and tolling bells, an apolitical atmosphere and the hourslong reading of the names of the dead.

For relatives, it’s an occasion to keep their loved one in the public’s consciousness, while also having a tone of personal remembrance. Some speakers updated their lost loved ones on weddings and grandchildren or described how their loss had moved them to connect with others who had been through tragedy.

Jerry D’Amadeo was 10 when he lost his father, Vincent Gerard D’Amadeo. The son said he worked this summer with children at Sandy Hook Elementary School in Newtown, Connecticut, where 20 children and six school staffers were massacred in 2012.

“Sometimes the bad things in our lives put us on the path to where we should be going — to help others as many have helped me,” he said.

Financial and other hurdles delayed the redevelopment of the Trade Center site early on, but now the 9/11 museum, three of four currently planned skyscrapers, an architecturally adventuresome transportation hub and shopping concourse and other features stand at the site. A design for a long-stalled, $250 million performing arts center was unveiled Thursday.

Around the Trade Center, lower Manhattan now has dozens of new hotels and eateries, 60,000 more residents and ever-more visitors than before 9/11.

Meanwhile, the crowd has thinned somewhat at the anniversary ceremony in recent years. But there’s been no sustained talk of curtailing the ceremony.

Cathy Cava, who lost her sister, Grace Susca Galante, has attended all 15 years.

“I will keep coming as long as I am walking and breathing,” Cava said, wearing a T-shirt with her sister’s photo.

“I believe most of her spirit, or at least some of her spirit, is here. I have to think that way.”