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.
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A wintry start to the weekend
A teacher has died after being hit by a dislodged manhole cover which smashed through her windscreen.
A dog has been found with its paws frozen to the ground in a New York park after being chained to a tree.