Resolve to never forget the horrors of Sept. 11 gripped those attending solemn Ground Zero ceremonies marking the 13th anniversary of the terrorist attacks that killed nearly 3,000 people.

A bell tolled for the dead Thursday morning, followed by readers reciting every name: those who died when hijacked planes felled the Twin Towers; in a similar attack on the Pentagon; in the crash of United Flight 93 into a Pennsylvania field; and the 1993 bombing of the World Trade Center.

A boy thanked the father he never met for giving him life. Some of those who lost wives or husbands choked back tears as they tried to put words to the holes in their hearts.

When a man uttered a prayer in a foreign tongue, no translation was needed.

"If this doesn't keep on happening, little by little people will forget," said Philip Hayes, 60, who has attended the ceremonies every year. "Sadly, this is our Pearl Harbor."

Hayes watched the towers fall on television from his home in Downey, California. His father, also named Philip, of East Northport, died while serving as the World Trade Center's fire safety director.

Many in the crowd in lower Manhattan wore light-blue ribbons pinned to their chests, signifying their status as someone mourning the loss of a relative. At one point, someone let go of a heart-shaped balloon, which soared past the gatherings inside and outside the Ground Zero plaza.

Dignitaries, including Gov. Andrew M. Cuomo, New Jersey Gov. Chris Christie and U.S. Secretary of Homeland Security Jeh Johnson, attended but did not speak.

The ceremony had ended by the time retired New York City firefighter Chris Eckhoff returned to Ground Zero for the first time in 13 years.

After Sept. 11, 2001, he spent almost two weeks searching for survivors, then victims' remains -- including those of fellow firefighters -- on the massive, smoldering pile of steel and rubble.

"I came today in hopes of easing the anxiety," said Eckhoff, 55, of Wantagh, shaking his head because his return only heightened the emotions he felt on 9/11. "It's rough right now. I saw a lot of stuff and lost several friends."

He and about 200 other first responders rode motorcycles Thursday morning from Long Island to Ground Zero, but Eckhoff declined invitations to visit the National September 11 Memorial & Museum.

"I'm not ready for that," he said. "I thought I would be, but this is going to take some real time."

Victims of the deadliest terrorist attacks in American history were also commemorated at the Pentagon, where President Barack Obama spoke at a wreath-laying ceremony without mentioning the ongoing war against extremists -- most recently the Islamic State in Iraq and Syria.

"We cannot erase every trace of evil from the world," Obama said. "That was the case before 9/11, and that remains true today."

In Shanksville, Pennsylvania, former House Speaker Dennis Hastert gave the flag that flew atop the U.S. Capitol on 9/11 to the Flight 93 National Memorial.

Thursday was the first time that the Sept. 11 museum ran the commemorative program. Another first was allowing the public access to the Ground Zero plaza, considered hallowed ground by 9/11 families.

From 6 p.m. to midnight, throngs spilled into the plaza, with its reflecting pools, where long-stem roses, photographs and notes adorned the engraved names of the victims.

Photographer Brian Prahl, 36, of Sunnyside, Queens, came with flowers also.

"I was fortunate that I did not lose anyone on 9/11, but I still feel it's important as a New Yorker to come here and reflect on the day innocent people died," he said.

Starting at sundown, the annual "Tribute in Light" -- two streams of blue light projected where the Twin Towers once stood -- shot skyward like beacons of renewal and hope.

Compared to past remembrances, there seemed to be a different mood yesterday, according to Queens resident Ian Tamayo, 32, who lost his engineer father on 9/11.

"There was a focus on celebrating the lives that were lost," Tamayo said. "Back then, we were in the pit standing on the dirt ground. It was raw. It is still raw, but now it looks more like a memorial -- a place to visit -- and it makes it a lot easier. Today, I remember my father's love for music. He was always singing and playing his guitar."

The remains of Nicholas Chiarchiaro's wife Dorothy and niece Dolores Costa were never found. Thursday, he made an emotional visit to the underground area where unidentified remains are kept. He remembered how his wife, just minutes before the attack, called him just to say "I love you."

"It was kind of overwhelming to go in there and know ... that part of my wife may be in there," said Chiarchiaro, 71, of Staten Island.

Two years after Sept. 11, he visited his ancestral home in Sicily, bringing along some family photos.

A relative took him into the family vineyard, dug a hole and placed the photos in the soil, telling the widower, "Your family will be here forever."

With AP