US investigators have released images of a person of interest in the fatal shooting of the influential conservative activist Charlie Kirk and said they had found the bolt-action rifle believed to have been used in the politically charged killing.

Kirk, a 31-year-old author, podcast host and close ally of US President Donald Trump, helped build the Republican Party's support among younger voters. He was killed on Wednesday by a single gunshot as he gave a talk at a university in Utah in what Trump called a "heinous assassination."

Investigators have yet to publicly discuss any motive, but Trump told reporters that he had an indication of the killer's motivation. "We'll let you know about that later," he said, adding that law enforcement was making "big progress" in the investigation.

FBI and state officials said the killer arrived on the campus a few minutes before the start of the event, a debate led by Kirk titled "Prove Me Wrong" outdoors in front of 3,000 people at Utah Valley University in Orem, Utah.

Security-camera videos show a person going up stairwells to get onto a roof before firing at Kirk, the officials told a press conference.

Kirk, a staunch defender of gun rights, was answering an audience question about mass shootings when the bullet struck his neck. Audience members fled in panic.

The shooter jumped off the roof and fled into an adjoining neighborhood, said Robert Bohls, the FBI special agent in charge.

Investigators found a "high-powered, bolt-action" rifle in a nearby wooded area, and were examining that along with palm prints and footprints for clues.

With classes canceled on Thursday, the roof of the building on the largely deserted campus and the woods were strung with yellow tape as investigators scoured them for evidence.

The shooter appears to be of college age and "blended in well" on the campus, Utah Public Safety Commissioner Beau Mason told reporters.

The shooter has not been publicly identified, though lawmakers, commentators and online sleuths have already filled social media and message boards with speculation and blame-casting about the killer's ideology.

The FBI offered a US$100,000 reward for information leading to the killer's arrest and circulated grainy images taken from security cameras showing a "person of interest" wearing a black top, black sunglasses and a dark baseball cap. The long-sleeved top was emblazoned with an image of a bald eagle flying across a US flag.

Utah state officials called the person in the photos "the potential shooter."

Trump said he would award Kirk the Presidential Medal of Freedom, the nation's highest civilian honor. Vice President JD Vance canceled his trip to New York to commemorate the attacks by al Qaeda on September 11, 2001, and instead traveled to Utah to see Kirk's family and to fly them and Kirk's casket home to Arizona aboard Air Force Two.

"Charlie Kirk was a great person, a great man – great in every way, especially with youth," Trump told reporters, saying he hoped that his killer would be apprehended soon. "Hopefully we'll have him and we will deal with him very appropriately."

Kirk began in conservative and right-wing politics as a teenager, a career he has described as shaped by his Christian faith. A little more than a decade later, some of the friends he made along the way are now at the highest levels of US government and media, with Vance recalling that he was in multiple group chats with Kirk.

"So much of the success we've had in this administration traces directly to Charlie's ability to organise and convene," Vance wrote in a tribute posted on social media. "He didn't just help us win in 2024, he helped us staff the entire government."

Kirk's organisation, Turning Point USA, said in a statement that its co-founder had believed in "the power of argument and good-faith debate" and had received thousands of threats. (Reuters)