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
Volkswagen could start recalling cars containing emissions cheating software in January, according to the firm's new chief executive.
A toddler survived for days on his own after his mother collapsed and died as she fed his one-year-old brother - who also choked to death.
A police officer has won a legion of fans after a picture of him comforting a baby girl caught up in a horror car crash went viral.