US investigators say they have found a bolt-action rifle they believe was used to kill conservative activist Charlie Kirk and released images of a "person of interest" as they searched for the shooter they described as "college age".

Kirk, a 31-year-old podcast-radio commentator and 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.

FBI and state officials said the killer arrived on the campus a few minutes before the event began, a talk by Kirk titled "Prove Me Wrong" outdoors in front of about 3000 people at Utah Valley University in Orem, Utah, about 65km south of Salt Lake City.

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