A single shot was fired at the Obama campaign office in the western US state of Colorado, damaging a window, a local media report said.
No one was injured, though people were inside the offices when the incident occurred Friday, Denver police spokeswoman Raquel Lopez said.
"It looks like it was one shot that was fired into the structure," she was quoted as saying by The Denver Post.
The police has a description of a "possible vehicle of interest," she said, adding that detectives are reviewing any available video footage of the incident and pursuing leads.
President Barack Obama was in Washington when the incident took place.
According to Politico, the Obama Campaign has some 30 field offices in this battle ground state of Colorado.
Obama campaign officials did not immediately comment on the incident, which comes as he and his Republican rival Mitt Romney battle into the final weeks before the November 6 presidential election.