Siller was one of 343 firefighters who died in the attacks. According to his family, he ran 1.7 miles through the tunnel with 60 pounds of gear strapped to his back to reach the Twin Towers after learning that a plane had crashed into the North Tower. Nearly 2,900 people died at Ground Zero that day.
His body was never recovered.
Siller left behind a wife and five children. His youngest son, Stephen Siller Jr., was an infant at the time of his father's death.
Hundreds of people joined the commemorative walk on Friday. After emerging from the tunnel on the Manhattan side, participants were joined by a 20-foot steel beam that has traveled across the country during the past four and a half months. The beam made stops in states including California, Florida and Texas, as well as the White House earlier this week.
Organizers said the effort aims to honor first responders and attack victims while educating younger generations about the Sept. 11 attacks.
The family then continued to FDNY Ten House, the fire station across from the World Trade Center site.
At the time of the attacks, Siller lived on Staten Island and worked out of a Brooklyn firehouse. He had just completed an overnight shift and was headed home to play golf with his brothers, according to his family, when he turned around after hearing about the crash at the North Tower.
His brother, Frank Siller, who founded the Tunnel to Towers Foundation, reflected on Stephen Siller's decision to respond that morning.
"It's very emotional. It's very emotional. You know that day, you know, he was called to duty. He could have come home and play golf with his other, with his brothers. And no one would have thought any of it because he would have got there later on and helped with the recovery effort. Like so many great heroes did." he said. "But that was not his. What? He was built. That's not how he was built. The ultimate act that he paid, has now changed the lives of thousands of heroes and their families that have similar, have lost a loved one."
Stephen Siller Jr. said the family's loss led to a mission that continues today through the foundation.
"25 years ago, our family's life was forever changed. Now, we suffered a deep tragedy that we didn't know what we would do with that loss, with that suffering," Stephen said. "Yet we were given this gift of the Tunnel to Towers Foundation and the work that we're able to do to honor my father to keep his memory alive."
The steel beam involved in the commemorative effort made 40 stops in 21 states before returning to New York, where the journey concluded Friday on the 25th anniversary of the attacks.
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