Uniformed officers filled the streets of New York City on Sunday, kicking off the 23rd annual Tunnel to Towers 5K Run and Walk to honor, remember and celebrate the lives of fallen American military service members and first-responders.

The event, symbolizing fallen FDNY firefighter Stephen Siller's final footsteps from the Battery Tunnel to the Twin Towers on 9/11, drew more than 40,000 people this year, a testament to the level of support for the cause.

"A friend of [Stephen's], Billy Codd, said to me one day, ‘Hey Frank, why don’t we have a run… Why don't we just do what Stephen did and run through the tunnel to the towers?'" Frank Siller, CEO of the Tunnel to Towers Foundation and brother to Stephen Siller, told Fox News' Abby Hornacek.

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T2T 5K

This year's Tunnel to Towers 5K drew over 40,000 people. (Fox News)

Then, the idea was born. 

"When he said that, I was overcome with emotion," he continued.

The inaugural event drew about 1,000 people and has increased massively in the two decades that followed.

Stephen Siller, Jr. — whose late father rushed the path that so many now commemorate — also spoke to Hornacek about the emotional impact of seeing so many people come out to support the cause.

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"It's a beautiful thing," he said. "I feel his presence in my life every day. Because of him, we're able to help so many people, and I'm so grateful for what we were able to make out of his loss."

"I think every single one of us is truly blessed." 

Frank Siller said the event goes beyond honoring those who made sacrifices on 9/11 and also pays respect to the thousands of military personnel who lost their lives in the war on terror as well as the police officers, firefighters and other first-responders who die daily across the nation.

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"We made a promise that we're going to take care of every single one of them and their families that are left behind, forever," he said.

Tunnel to Towers is a charitable organization that assists Gold Star families and families of fallen first-responders by providing them with mortgage-free homes.