Barack Obama. Photo by Reuters
Campaign workers were inside but no one was struck.
Denver Police Detective Raquel Lopez said a worker inside the office called police in mid-afternoon to report that a bullet had shattered a window pane.
"Right now, no one is in custody and investigators are looking at surveillance video to see if a vehicle can be identified," Lopez said.
Lopez said it was a single shot and no one inside was physically injured.
Detectives are not aware of any threats made to the campaign, but are exploring all possibilities, she said.
