'We're a tinderbox. We are seeing more support for violence'

Mr Trump said he will award Mr Kirk a posthumous Presidential Medal of Freedom, the highest civilian honour in the US. The president made the announcement at a ceremony honouring those killed during the September 11 terrorist attacks.

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The gunman had calmly blended in with students at Utah Valley University in Orem before climbing on to a rooftop and firing the shot that killed the right-wing influencer in front of horrified onlookers.

Investigators believe the killer arrived on campus at 11.52am — some 40 minutes before the assassination on Wednesday — and “appears to be of college age”.

Beau Mason, Utah’s Department of Public Safety Commissioner, said: “After the shooting, we were able to track his movements as he moved to the other side of the building, jumped off of the building and fled off of the campus and into a neighbourhood.

“Our investigators have worked through those neighbourhoods, contacting anybody they can with doorbell cameras."