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MORRISON, Colo. — Monday marked the 16-year anniversary of the terrorist attacks on Sept. 11, 2001.

On the day of remembrance, about 2,000 people gathered at Red Rocks Amphitheatre for the annual 9/11 stair climb. It’s a way to honor the firefighters who died on that day.

There was a moment of silence at 8:46 a.m. and the climb began at 9:03 a.m.

The stair climb is the largest in the country. It’s the equivalent of the 110 stories of the World Trade Center — nine laps around the amphitheater.

It honors the 343 firefighters who died that day.

“I can’t even imagine showing up on the first engine and seeing what they saw and going up those flights of stairs,” Lafayette firefighter Kenny Storwold said. “Most of them knew they wouldn’t come out.”

The climb benefits programs that support families of fallen firefighters.

Registration costs $35.