DALLAS -- Each chime ringing through the Renaissance Tower in downtown Dallas Saturday was an echo from the past.
The bell signified a first responder making it to the top of the tower.
Firefighters, police officers and paramedics from all over the country make the 52-story climb twice.
It's a symbolic tribute to the 110-story trek made by first responders in the World Trade Center on September 11, 2001. Sunday marks the 15th reunion of the worst terror attack in U.S. history. More than 400 first responders died that day.
The 9/11 Memorial Stair Climb is in its 6th year. This year, participants wore tags of fallen comrades proudly up each flight of stairs.
"God bless all of them," said Cap. Tom Wendling from Highland Park DPS. "It's special."
They're exhausted after the climb. Firefighters participating wore full gear up each flight. But they are all driven by a common understanding of the sacrifice.
"As exhausting as it is, I can't imagine what they went through that day," said Mari Sanchez with the Dallas Fire-Rescue Department.
"To see them doing this in remembrance is very powerful," said supporter Brandy Runyan.
Philip Townsend / WFAA