Paul Berger, Jim Hook and John Bacon

USA TODAY Network

NEW YORK — Tears, vows and memories marked emotionally charged ceremonies Sunday at the World Trade Center site, the Pentagon and a rural field in western Pennsylvania on the 15th anniversary of the most deadly terror attack in U.S. history.

Bells tolled across much of the nation at 8:46 a.m. ET, the moment the first plane struck the North Tower of the World Trade Center. Thousands gathered here as family members, after a moment of silence, solemnly said aloud the names of the almost 3,000 victims. The presenters each read about 30 names, ending with a few words about their own loss.

“I love and miss you dearly, and I wish I was able to spend more time with you,” said Maria Frances Pullis, who was only 17 months old when her father, Edward, died in the devastation at Ground Zero. “But I know you’re up in heaven watching.”

Presidential hopefuls Hillary Clinton and Donald Trump attended the ceremony. Both left well before the almost four-hour program ended, with Clinton leaving after feeling "overheated," her spokesman said.

But this event wasn't about them. Lionel Keaton traveled by bus from North Carolina with 50 relatives to pay homage to his niece Tamitha Freeman, who died when the South Tower collapsed, spewing steel, glass and dust across Lower Manhattan.

"We just decided as a family since it was the 15th anniversary that we would all get together and make a bus trip to New York to celebrate her life," Keaton said.

At the Pentagon, President Obama participated in a wreath ceremony and paid tribute to the 184 victims killed when American Airlines Flight 77 slammed into the western side of the massive building.

"No deed we do can ever truly erase the pain of their absence," Obama said. "Your steadfast love and faithfulness has been an inspiration to me and our entire country."

Defense Secretary Ash Carter vowed to hunt down all who attack Americans.

“Wherever they are, they will surely, no matter how long it takes, come to feel the righteous fist of American might,” Carter said.

In Pennsylvania, Gordon Felt lost a brother on United Airlines Flight 93, which had flown out of Newark bound for San Francisco. The 9/11 Commission determined the hijackers had turned the plane around and were heading for a target in Washington, D.C. Some passengers fought for control of the plane, which crashed more than 150 miles northwest of Washington.

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Forty victims on the plane were killed. "Patriotism and heroism" were on display that day, Felt said. "We can't leave the events of Sept. 11 behind," Felt said. "It will always be bittersweet for me."

Tom Rooney, president of Rooney Sports, and some of the Pittsburgh Steelers football team comforted Flight 93 families 15 years ago and raised money for the memorial there.

"The first shot in the war on terrorism was fired by those who wrestled the control of the plane, brought it down and saved the Capitol," Rooney said Sunday. "We feel almost like war veterans coming back to a war memorial."

Boy Scouts from Troop 262 voted in August to visit the Flight 93 Memorial. "It's so tragic," said Eagle Scout Marty Zender, who was just 2 in 2001. "We want to remember, to repay and respect them."

Korean War veteran Leslie Wilson, 80, walked with two canes as he followed his two grandchildren to the Flight 93 Memorial.

"I think they need to know these important things that happened in the United States," he said. "If we don't teach them, who's going to teach them."

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In New York, actress Renée Elise Goldsberry, of the Broadway show Hamilton, sang Forever Young. The names of the 2,977 people who died that day, plus the six killed in the 1993 World Trade Center bombing, were read as they are every year. The solemn program paused several times to mark key moments from that morning, including when the North Tower fell at 10:28 a.m.

Joe Quinn, whose brother Jimmy died in the North Tower, then addressed the crowd. Quinn, who enlisted in the Army and served in Iraq and Afghanistan, said that as hard as the weeks after 9/11 were, he misses the national sense of unity of those days. He urged relatives of those who died in the terror attacks to switch off their TV sets, go outside and get involved in their community.

Crowds for the ceremony have diminished over the years. Some years the crowd dwindles to 100 people by the end.

“Parents of the deceased are getting older, younger people usually can’t make it because of work obligations,” said Tom Acquaviva of Wayne, N.J., whose 29-year-old son, Paul, a father of two, died on 9/11.

Acquaviva said he thinks of Paul daily, but the anniversary of the attacks remains a special day.

“My wife and I lost everything,” Acquaviva said. “You carry on, but you don’t move on.”

Hook reported from Shanksville, Pa.; Bacon from McLean, Va.

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