Denver officials said they are not aware of any threats made to President Barack Obama's campaign, but are exploring all possibilities as they investigate the shooting at his re-election headquarters.
Right now, no one is in custody and investigators are looking at surveillance video to see if a vehicle can be identified.
More top news
A police officer who spent six days crawling the London Marathon in a gorilla costume has finally completed the 26-mile route.
Nicola Sturgeon calls on voters to make sure "Scotland's voice is heard" and not "dragged back by the Tories" in upcoming vote.
Strictly Come Dancing star Louise Redknapp has dismissed speculation she has separated from former footballer Jamie as "completely untrue".