The FBI is offering a $100,000 reward for information leading to the identification and arrest of the gunman who shot conservative influencer Charlie Kirk.

They have released security camera images of a ?person of interest? ? a slim, young man with dark hair, wearing a baseball cap and black T-shirt. Investigators said the gunman appears of ?college age?.

They said they have tracked the suspect?s movements on campus at Utah Valley University on Wednesday, where the shooting took place.

Kirk, 31, who founded conservative youth political group Turning Point USA, was shot in the neck while speaking to students.

FBI special agent in charge Robert Bohls said they had recovered the suspected weapon ? an imported Mauser .30-06 bolt action rifle ? in a wooded area near the scene wrapped in a towel, as well as a palm print and a ?footwear impression?.

He told a news conference on Thursday they do not think there is a risk to the community in the city of Orem, 40 miles (64km) south of Salt Lake City.

Utah Department of Public Safety Commissioner Beau Mason said the gunman ? who has not been publicly identified ? ?appears to be of college age?, and had blended well into the campus environment.

Investigators say the suspect arrived on campus at 11:52 local time on Wednesday ? just under half an hour before Kirk was killed with a single shot at around 12:20.

The gunman reached his position by climbing the stairwells to the roof and then making his way across to the shooting location. After opening fire, he jumped off the building and escaped, officials said.

A motive for the shooting is still unknown, though Utah Governor Spencer Cox called it a ?political assassination?.

Kirk, who frequently held debates and speaking events on university campuses, was addressing a crowd of about 3,000 people and discussing gun violence in the US in the moments before he was shot.

?Do you know how many transgender Americans have been mass shooters over the last 10 years?? an audience member asked at the event, titled Prove Me Wrong.

?Too many,? replied Kirk.

The questioner said there had been five in 10 years and asked Kirk how many mass shooters there had been in that time.

?Counting or not counting gang violence?? Kirk replied before gunfire rang out.

He was pronounced dead at a local hospital hours later.

He leaves behind a wife, Erika, and two children, a one-year-old son and three-year-old daughter.

People later gathered near campus hoping to retrieve vehicles and other possessions left behind as they had fled the scene.

?It?s scary,? student Brock Anderson told the BBC. ?It?s unsettling around the campus right now.?

Vice-President JD Vance, a close friend of Kirk, and Second Lady Usha Vance flew to Salt Lake City on Thursday to retrieve his coffin.

They will travel with Kirk?s family and some of his friends as they fly the casket back to the activist?s Arizona hometown on Air Force Two.

The Vances had initially been due to visit Ground Zero in New York City on Thursday morning to commemorate the 24th anniversary of 9/11.

After attending a 9/11 memorial service at the Pentagon on Thursday, President Donald Trump expressed his condolences and said: ?You don?t replace a Charlie Kirk.

?He?s unique, as you all know,? added the president, who has himself survived two attempts on his life.

He had initially been scheduled to give a 9/11 memorial address outside the Pentagon, but the BBC?s US partner, CBS News, reports the location was switched to inside the building out of caution, because Kirk?s killer is still at large.

Trump said he intends to posthumously award Kirk the Presidential Medal of Freedom, the nation?s highest civilian honour.

The president said he would call Kirk?s family this afternoon. (BBC)

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