Colorado firefighters started an annual, nationwide 9/11 memorial.
Seven years ago, a group of firefighters in Denver turned a training exercise into a memorial. They decided to climb 110 stories, the height of the twin towers,in memory of the 343 FDNY responders who died on 9/11.
They decided to make it an annual event, and each year more people joined. They've capped the number of climbers at 343, so that each climber carries a picture of a fallen FDNY comrade.
Oren Bersagel-Briese was one of the original climbers. He says wearing full fire gear makes it physically challenging, but that doesn't compare to the emotional impact.
"We hear all the time about guys who get in touch with the family of the firefighter that they climbed for," he says, "The connections that are made that way are just unbelievable to let them know we don't forget the sacrifices they continue to make on a daily basis and that their loved one made for everybody on September 11."
The event has spread to 34 states and there have also been international stair climbs. Some stair climbs are open to the public, but in Denver it's only firefighters.
Bersagel-Briese says even though a lot of climbers don't have personal ties to 9/11, the climb allows them to actively remember the tragedy.