The man accused of assassinating US conservative activist Charlie Kirk was afraid of being shot by police and agreed to surrender only if it was done peacefully, a sheriff involved in the arrest says.

Tyler Robinson appeared quiet and sombre when he arrived with his parents to turn himself in last week at the Washington County Sheriff's office, said Sheriff Nate Brooksby.

It was a day after Kirk was shot and killed at Utah Valley University on September 10.

"He didn't want a big SWAT team at his parents' house or his apartment," said the sheriff, who was only involved with the surrender and not the broader investigation.

"He was truly fearful about being shot by law enforcement."

Prosecutors on Tuesday charged the 22-year-old Robinson with capital murder and announced they will seek t

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