On the morning of Sept. 11, 2001, 34-year-old New York City firefighter Stephen Siller had just finished his overnight shift when he heard a call come in on the scanner.
"He immediately knew something was awry," so he drove to his firehouse in Brooklyn, his son, Stephen Siller Jr., told ABC News.
"When he got there, he realized his whole unit had responded [to the World Trade Center]," Siller Jr. said. "So he grabbed his gear, threw it in his truck and started to make his way to the World Trade Center. But on 9/11, they shut down every entryway into Manhattan."
He parked his car by the Brooklyn Battery Tunnel, Siller Jr. said, put about 60 pounds of gear on his back, and ran through the tunnel to the South Tower of the World Trade Center.
Stephen Siller was among 343 New York City Fire Department members killed on 9/11. He was survived by his close-knit siblings, his wife Sally, and his five children, including Stephen Jr., who was 9 months old.
Siller Jr., now 25, doesn't have his own memories of his father, but he's poured his energy into honoring those who died on 9/11. And to commemorate the 25th anniversary of the terror attacks, he spent the last four months bringing a 16,900-pound steel beam from the World Trade Center to dozens of cities across the U.S. in hopes that seeing a piece of history up close will help Americans remember, learn and be inspired.
The Steel Across America campaign allows the public to "put their hand on that steel and feel the loss and sacrifice that was made that day -- the sacrifice of first responders and their loss, the loss of innocent people who just went to work," Stephen Siller Jr.'s uncle, Frank Siller, told ABC News.
Siller Jr. described the entire tour as "incredibly powerful."
At the first stop, ground zero, a firefighter whose entire unit was killed on 9/11 "walked up to the steel and he put his hand on it, and he was overcome with emotion," Siller Jr. recalled. "You could just immediately see tears rushing down his face. And he started to hug the steel like he was hugging his friends he lost 25 years ago."
To Siller Jr., the steel represents the more than 1,000 bodies never recovered from the World Trade Center -- including his father's.
"Who they are is ingrained in that steel, what they did is ingrained in that steel. It holds the memory of that day," he said. "It's the most overwhelming piece of history I've ever come across."
The beam represents "resilience, strength, but also sacrifice," Frank Siller added. "... Ground zero is my brother's burial place, obviously. But the steel has his -- embodies his soul, and the souls of so many."
As Siller Jr. spent his summer traveling the U.S., from Oklahoma to California to Mount Rushmore in South Dakota, he said he was surprised by how many people had personal connections to the terror attacks, like the police officer he met in Seattle whose uncle was killed on 9/11.
"And we had thousands of people show up to 35 events across the country who may not have had a connection to 9/11," he added, "but for them, they wanted to mourn that day, they wanted to make sure they never forgot."
"Everywhere we went was incredible -- the support, it was mind-boggling," Siller Jr. said. "In Texarkana, a small, little town in Texas, we had about 1,000 people show up to an event. It was unbelievable. You know, companies letting all their employees out of work to go see this piece of steel."
Siller Jr. found one of the most memorable stops to be Emma E. Booker Elementary School in Florida, where President George W. Bush was reading to students on the morning of Sept. 11, 2001, when he learned of the attacks.
"We normally have stairs that go up to the steel truck," Siller Jr. explained, but on this May visit to the school, it was pouring rain.
Still, the children "were begging the firemen, 'Hey, can we get up? We just want to touch the steel,'" Siller Jr. said.
As the kids were lifted up, they began "praying over this steel," he recalled.
"Little kids who understand that, you know, this is a sacred piece of history ... it once held up the Twin Towers, it once held floors and walls, and now it holds the gravity of that day," he said.
"I was really shocked at how interested they were," he said.
To Frank Siller and Siller Jr., educating the next generation was a key goal of the Steel Across America campaign.
"These kids were incredible; they were patient ... and they consistently showed up," Siller Jr. said. "No matter where we were in the country, there were families there, which was beautiful -- fathers and mothers wanting to teach their kids."
"I was 9 months old on 9/11 -- if you were born after me, and not in New York City, you know, there's a high chance that you have no real understanding of that day," he continued. "It's important that that lesson is taught to these kids -- there's so much to be pulled from the sacrifices made."
The Steel Across America campaign is part of the Tunnel to Towers Foundation, started by Frank Siller in honor of his brother's run from the Brooklyn Battery Tunnel to the World Trade Center. The foundation supports military members, first responders and their families across the U.S. The foundation is currently providing wounded veterans and first responders with mortgage-free, specially-adapted smart homes.
"We didn't let Stephen just perish in that rubble and let that story end there. No, we took his act of courage and sacrifice and said, 'We're going to do good with it,'" Frank Siller said.
"Would Stephen make that run again through the tunnel, knowing he was gonna die? I think he knew he was gonna die," Frank Siller said. "At the very least, he knew it was a strong possibility that he might not be coming home. And to think that he did that, knowing how much he loved Sally and his kids, people like that are few and far between. ... And it's got to be recognized and never forgotten."