Officers went floor by floor to clear the Alliance Center, a building that houses several environmental organizations and provides shared workspaces.
Other buildings were locked down, and people were asked to stay away from the area.
Nicole Arnone, who works for the nonprofit Rocky Mountain Institute on the third floor of the building, said she was listening to a conference call with earbuds when a co-worker tapped her on the shoulder and said, "We just heard a few loud bangs and some screaming, and I think we need to go." "We debated whether to hide in the corner or run," Arnone said, and they decided to flee.
She smelled smoke and thought it was either a fire or gunpowder, she said.
Arnone spotted a shell casing in the stairwell as she fled.