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
Danny Boyle's Trainspotting 2 replaces the tagline "Choose Life" with "Choose Facebook, Twitter, Instagram".
Settled and very cold across the UK this weekend with a widespread, overnight frost.
BT has announced it is increasing prices for broadband as well as some calls and is to begin charging TV customers for BT Sport.