Kirk shooting suspect not co-operating with authorities

The man arrested in the killing of right-wing activist Charlie Kirk is not co-operating with authorities, but investigators are working to establish a motive for the shooting by talking to his friends and family, Utah Governor Spencer Cox says.

Cox said accused gunman Tyler Robinson, 22, would be formally charged on Tuesday. He remains in custody in Utah.

Investigators are yet to piece together why Robinson allegedly scaled a rooftop at Utah Valley University during an outdoor event and shot Kirk in the neck at long range on Wednesday.

Kirk, a staunch ally of President Donald Trump and co-founder of the conservative student group Turning Point USA, was killed by a single rifle shot during the event attended by 3000 people in Orem, about 65km south of Salt Lake City.

The killing ushered in newfound fears of a spike in political violence in the United States and an ever-deepening divide between the left and the right.

Robinson has not confessed to investigators, Cox told the ABC program This Week.

"He is not co-operating, but all the people around him were co-operating, and I think that's very important," the Republican governor said.

One person who is apparently talking to investigators is Robinson's roommate, who was also a romantic partner, Cox said, citing the FBI. Cox described the roommate as "a male transitioning to female" and said the roommate has been "incredibly co-operative".

Reuters has not been able to locate the roommate, or representatives for the roommate, to seek comment. Reuters could not determine who is serving as Robinson's legal representative.

Asked on CNN's State of the Union program whether the roommate's gender identity is relevant to the investigation, Cox said that is what they are trying to figure out.

Investigators found messages engraved into four bullet casings, which included references to memes and video game in-jokes.

One of the inscriptions, according to an affidavit filed by authorities, read: "hey fascist! CATCH!" followed by a combination of directional arrows, an apparent reference to a sequence of button presses that unleashes a bomb in a popular video game.

Another casing, according to the affidavit, read, "If you read This, you are GAY Lmao," short for "laughing my ass off".

Kirk's charged rhetoric, which often involved anti-LGBTQI and anti-immigrant comments, attracted legions of conservatives but also engendered strong feelings from liberals and drew widespread criticism.

Robinson, a third-year student in the electrical apprenticeship program at Dixie Technical College, part of Utah's public university system, was taken into custody at his parents' house, about 420km southwest of the crime scene after a 33-hour manhunt.

Relatives and a family friend alerted authorities that he had implicated himself in the crime, Cox said previously.

While Robinson was raised by religious parents in a deeply conservative region of the state "his ideology was very different than his family", Cox said on Sunday on NBC's Meet the Press program, without going into specifics.

State records show Robinson was a registered voter but not affiliated with any political party. A relative told investigators that Robinson had grown more political in recent years and had once discussed with another family member their dislike for Kirk and his viewpoints, according to the arrest warrant affidavit.

Robinson was "not a fan" of Kirk's, Cox said on Sunday.

The killing has stirred outrage among Kirk's supporters and condemnation of political violence from some across the ideological spectrum.

Many Republicans, including Trump, have been quick to lash out at the political left, accusing liberals of fomenting anti-conservative vitriol that would encourage violence - even as the president and his allies have often invoked violent imagery against their opponents.

"The problem is on the left," Trump told reporters on Sunday. "A lot of people that you would traditionally say are on the left ... (are) already under investigation."

Mike Johnson, the Republican speaker of the US House of Representatives, criticised Democrats but also urged calm.

"There's this recognition that people have got to stop framing simple policy disagreements in terms of existential threats to our democracy," he said on the Fox News Sunday program.

On Meet the Press, Cox assigned some blame to social media, saying it has "played a direct role in every single assassination and assassination attempt that we have seen over the last five, six years".

Trump has credited Kirk with driving young voters to conservatism. His Turning Point movement says it has more than 800 chapters across college campuses. Kirk's widow on Friday said the movement's efforts would go forward.

A memorial event for Kirk will be held on September 21 in Glendale, Arizona, his organisation said.