The FBI has released images of a "person of interest" in the murder of popular conservative influencer Charlie Kirk, who was shot at a college event on Wednesday, as US President Donald Trump urged a peaceful response to the 31-year-old's killing.

The images show a young man believed to be of college age wearing a baseball cap, sunglasses and black T-shirt going up a flight of stairs. The FBI is offering a reward of up to $100,000 for information about the individual.

Utah Department of Public Safety Commissioner Beau Mason said on Thursday that, according to unreleased video footage, the suspect arrived at Utah Valley University at 11:52 am, and climbed up stairwells to the roof of a building overlooking the event.

As Kirk was speaking to a crowd of about 3,000 at the campus around 12:20 pm as part of his "American Comeback" tour, a shot rang out. Video shows Kirk clutching his neck and collapsing, bleeding profusely from the neck.

He was taken to a hospital where he was pronounced dead.

Video taken at the event showed what appeared to be a man on a roof fleeing after the shooting.

Salt Lake City FBI Special Agent Robert Bohls said that a "high-powered bolt action rifle," believed to be the gun used to kill Kirk, was recovered in a nearby wooded area through which investigators suspect the gunman fled.

Investigators said they are "exhausting every lead."

"We will not stand for what happened yesterday," Mason said. "We are investing everything we have into this, and we will catch this individual."

Trump, who soon after the killing angrily pledged a wide-ranging response against the "radical left", told reporters that Kirk had been "an advocate of nonviolence."

"That's the way I'd like to see people respond," he said.

Kirk, a conservative activist and close Trump ally, played an influential role in rallying young Republican voters. He launched the conservative organization Turning Point USA in 2012 at the age of 18, targeting a younger audience, and often made appearances at liberal-leaning campuses. He had a popular podcast and millions of followers on X.

Utah Governor Spencer Cox told reporters that Kirk's killing was "a political assassination."

"We just need every single person in this country to think about where we are and where we want to be, to ask ourselves: Is this what 250 years has wrought on us?"

Investigators said it appeared to be a targeted killing, and believe the shot had come from a rooftop, fired by someone dressed in black.

Police believe no one else was involved. Two people held in custody on Wednesday were released after questioning.

Kirk's assassination leaves the nation to reckon with violence that has become increasingly political and public. At the same time, it's forcing universities to question whether more security is needed to protect people speaking their minds to campus audiences.

"It's going to rattle college campuses," said Nico Perrino, executive vice-president for the Foundation for Individual Rights and Expression. "Colleges are going to be concerned about these sort of events moving forward, particularly if they are outside."

Most likely it will spur colleges to move large outdoor events into campus buildings, said Perrino, who is a frequent campus speaker himself. That's already the norm for most controversial visitors, who are typically hosted in auditoriums or classrooms that are easier to protect, he said.

Students at Utah Valley University on Thursday expressed shock and their broader fears as political divisions deepen across the country.

Dave Sanchez told AFP witnessing the killing made him "sick to my stomach".

"We watch him all the time and so it really does feel like one of your own family members, your own brother's been killed," said Sanchez, 26.

On Thursday, America marked the 24th anniversary of the deadly attacks with ceremonies in New York, at the Pentagon and in Shanksville, Pennsylvania. The commemorations were punctuated by moments of silence, the tolling of bells and the reading of the names of the nearly 3,000 killed.

The assassination of Kirk a day earlier prompted additional security measures at ground zero, where FBI Director Kash Patel was among the dignitaries in attendance.

The Associated Press contributed to this story.

shiguang@chinadailyusa.com