A man cleans up broken glass after a bullet was fired into a Denver campaign headquarters of President Barack Obama.
Although staff were in the building at the time of the Friday shooting, no one was hurt.
Denver police are looking at surveillance footage to possibly identify a passing vehicle.
The shooting comes as the presidential race enters its last few weeks.
Obama, who struggled during the first debate, is neck and neck with his Republican opponent.
But Vice President Joe Biden's energetic performance in his debate has reinvigorated liberals.
Republican running mates Mitt Romney and Paul Ryan are banking on the United States' embattled economy to boost their numbers ahead of the November polls.
