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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The city has endured weeks of disruption as police investigated the poisoning of Sergei and Yulia Skripal.
Dr James Goodman underwent the procedure on his tongue at the Royal Marsden NHS Hospital.
The boat is carrying 224 migrants the Italian coastguard had assigned to Libyan authorities to rescue.