Police and US federal agents say they have found a bolt-action rifle they believe was used to kill conservative activist Charlie Kirk during a university appearance in Utah but are still hunting the shooter.

Kirk, a 31-year-old podcast-radio commentator and a close ally of US President Donald Trump, is credited with helping build the Republican Party's support among younger voters.

He was killed on Wednesday by a single gunshot in what Utah Governor Spencer Cox called a political assassination.

The killing, captured in graphic detail in videos that rapidly spread around the internet, occurred as Kirk spoke onstage at an outdoor event called "Prove Me Wrong" in front of about 3000 people at Utah Valley University in Orem, Utah, about 65km south of Salt Lake City.

The killer arrived on c

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