Exactly 343 American flags — carried by 343 current FDNY personnel — looped around the pews of St. Patrick’s Cathedral Saturday afternoon: one flag for each firefighter killed in the 9/11 attacks.

The flagbearers stood in position for more than 90 minutes as families of the 343 and others whose lives changed that Tuesday morning commemorated 9/11’s 15th anniversary.

“This is how we heal. Fifteen years is not 15 years, it’s yesterday to us,” said Ray Pfeifer, an ill retired FDNY firefighter from Hicksville who was key in lobbying Congress to maintain health benefits for sick 9/11 responders.

President Barack Obama sent a letter read from the pulpit by his attorney general, Loretta Lynch, in which he urged, “may we never forget the stories of the fallen.”

“America endures because of those who believe in the promise of our nation and are willing to give their lives to protect it,” she read.

The service fused Judeo-Christian traditions: a Jewish cantor sang the Kaddish mourner’s prayer at the beginning of the service, and Cardinal Timothy Dolan, the archbishop of New York, delivered a final blessing, devoting his prayers to first responders “who keep us safe from another 9/11.”

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New York City Mayor Bill de Blasio noted that 9/11 was not only the deadliest day in FDNY’s 150-year history but the deadliest for firefighters in U.S. history.

“They did things that day that most of us could not fathom. They rushed to danger not from it. They carried others on their backs, and never turned their backs on others,” de Blasio said.

The FDNY said 125 of the firefighters who died had Long Island connections.

John Garret McNamara, the son of Firefighter John McNamara from Engine 234, who died in 2009 of 9/11-related cancer, urged mourners to remember his dad.

“My dad was a hero,” the young boy said, “and I hope people don’t forget him.”

McNamara was one of 127 who have died since the attacks of cancers believed to be connected to Ground Zero’s toxins. Those names and the 343 who died in attacks’ immediate aftermath were read aloud in the cathedral Saturday. Even when recited at a rapid clip, the “scroll of honor,” as the terrible roster is called, took nearly a third of the service.