NEW YORK — The U.S. marked the 15th anniversary of 9/11 with the solemn roll call of the dead but couldn’t keep the Presidential campaign from intruding on what is traditionally a politics-free moment of remembrance.

Hillary Clinton left about 90 minutes into the Ground Zero ceremony after feeling “overheated,” her campaign said.

Video showed her knees buckling as three people helped the 68-year-old Democrat into a van in the muggy, 80-degree heat. Later in the day, she said she was “feeling great” as she walked to a vehicle.

Donald Trump, who has repeatedly questioned whether Clinton is physically fit to be President, was also at the ceremony for a time and left after she did. Asked about the incident, the Republican nominee said only: “I don’t know anything about it.”

The episode cast a political shadow over an event that has tried to keep the focus on remembrance by inviting politicians but barring them from speaking. The two candidates had followed the custom of suspending all TV ads for the day.

The politics of the moment weren’t entirely absent from the ceremony, where some victims’ relatives pleaded for the nation to look past its differences, expressed hopes for peace or called on the next Commander-in-Chief to ensure the country’s safety.

Joseph Quinn, who lost his brother, Jimmy, appealed to Americans to regain the sense of unity that welled up after the terror attacks.

“I know, in our current political environment, it may feel we’re divided. Don’t believe it,” said Quinn, who added that he served in the military in Iraq after Sept. 11. “Engage with your community. … Be the connection we all desperately need.”

Nearly 3,000 people died when terrorists slammed hijacked planes into the World Trade Center, the Pentagon and a field near Shanksville, Pennsylvania, on Sept. 11, 2001.

Organizers estimated 8,000 people gathered Sept. 11 at the lower Manhattan spot where the twin towers once stood. They listened to the nearly four-hour recitation of the names of those killed.

“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.

For Dorothy Esposito, the passage of 15 years feels “like 15 seconds.” Her son, Frankie, was killed.

About 1,000 people gathered for a name-reading observance in Shanksville. At a Pentagon ceremony, President Barack Obama praised military members and others who have helped the U.S. fight terrorism, urged Americans not to let their enemies divide them and called the country’s diversity one of its greatest strengths.

“We stay true to the spirit of this day by defending not only our country, but also our ideals,” he told hundreds of service members, survivors and victims’ relatives.

In New York, some victims’ relatives said their loss had inspired them to help others.

Jerry D’Amadeo said he worked this summer with children at Sandy Hook Elementary School in Newtown, Connecticut, where 26 children and adults were massacred in 2012.

“Sometimes the bad things in our lives put us on a path to where we should be,” said D’Amadeo, who was 10 when he lost his father, Vincent.

James Johnson was at Ground Zero for the first time since he last worked on the rescue and recovery efforts in early 2002, when he was a New York police officer.

The 9/11 museum and memorial plaza, three skyscrapers and an architecturally audacious transit hub have been built on land that was a disaster zone when he last saw it.

“I’ve got mixed emotions, but I’m still kind of numb,” said Johnson, now a Police Chief in Forest City, Pennsylvania. “I think everyone needs closure, and this is my time to have closure.”

Cathy Cava, on the other hand, has attended all 15 anniversary ceremonies since she lost her sister, Grace Susca Galante.

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

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By JENNIFER PELTZ and VERENA DOBNIK. AP writers Lisa Lerer, Tom Hays, Michael Balsamo and Deepti Hajela contributed