Police said a shot was fired through the window of President Barack Obama's Denver campaign office.
Police spokeswoman Raquel Lopez said people were inside the office when the shooting happened on Friday afternoon, but no one was injured.
A large panel of glass was left shattered at the office on West Ninth Avenue near Acoma Street.
Ms Lopez said investigators are looking at surveillance video but have not yet confirmed a description of a vehicle that might be linked to the shooting. Police did not immediately release other details while detectives pursue leads.
Ms Lopez said she was not aware of any previous threats against the campaign office.
The Secret Service referred questions about the incident to Denver Police.
An Obama campaign spokeswoman declined to comment.