Thousands took off Sunday morning on the annual Stephen Siller Tunnel to Towers Run and Walk commemorating the Sept. 11, 2001, terrorist attacks, just more than a week after a series of bombings rocked Manhattan and New Jersey.

The runners included about 100 people unable to race in Seaside Park, New Jersey, on Sept. 17 after a bomb went off.

The mood was festive on a crisp Sunday morning as runners started at the Brooklyn Battery Tunnel with former Mayor Rudy Giuliani sending them off and people gathered at the finish line.

Laura Marko, 50, of Toms River, New Jersey, was about to start the race in Seaside Park eight days ago when she saw smoke. Re-registering for the Tunnel to Towers on Sunday was a no-brainer.

“It really is our new normal, it’s ridiculous. And I never thought a small event like that would be disrupted. I never even thought it was real,” she said of the bombing.

Marko, who said her daughter is a cadet at West Point, said they ran the Tunnel to Towers race last year as well.

“I have absolute confidence in the NYPD and our military,” she said about running Sunday. “I feel great, I’m not afraid at all.”

As runners made their way out of the tunnel on the Manhattan side, more than 300 first responders stood, some high-fiving runners and most holding photos of firefighters who died on Sept. 11, 2001.

Later, a moment of silence was held for everyone who died that day, those sickened after working at Ground Zero, those who went to war after the attacks, and law enforcement officers killed defending the city.

Frank Sinatra’s “New York, New York” then played, the large crowd singing the chorus.

The race, in its 15th year, was named for Stephen Siller, 34, an FDNY firefighter who was on his way home when he heard of the attacks on the World Trade Center in lower Manhattan.

Siller, raised in Rockville Centre and a graduate of St. Agnes High School, turned around and drove his truck to the entrance of the Battery Tunnel, but it was closed, according to the foundation. He strapped on 60 pounds of gear and ran through the tunnel, dying during the rescue efforts.

The run benefits the Building for America’s Bravest program, which builds smart homes for injured service members.

“This year, especially, felt more poignant,” said Frank Vogt, 40, Siller’s nephew. “I was able to look back and reflect.”

Vogt, who lives in upstate Cornwall, said he’s now helping his children — 7 and 4 years old — understand what happened.

“They’re asking questions,” he said. “They have their own way of working through it.”

Justin Siller, Stephen Siller’s nephew, said what’s more amazing than the 15th anniversary of the run is that it keeps getting bigger every year.

“It means a lot just to see the amount of people that come and show support,” said Justin Siller, 35, who lives in Farmingdale, New Jersey. “It stinks that we lost him, but it wasn’t for nothing.”

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