The FBI says a weapon it believes was used to kill popular American right-wing influencer Charlie Kirk has been found, however the killer remains at large.

The 31-year-old conservative activist was shot while addressing a crowd of about 3,000 people at Utah Valley University in Orem, south of Salt Lake City, just after midday on Wednesday, local time.

FBI Special Agent in Charge Robert Bohls told the media the weapon believed to have been used in the shooting was a high-powered bolt action rifle.

"That was recovered in a wooded area where the shooter had fled," he told the media in a press conference on Thursday, local time.

He said investigators also had images of the suspect, and were analysing footprints and "other imprints".

Police and federal agents mounted an intense manhunt on

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