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 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.

In his NBC appearance, Cox confirmed that Robinson’s roommate “was indeed a boyfriend who is transitioning from male to female”.

Asked on CNN’s State of the Union program whether the roommate’s gender identity was relevant to the investigation, Cox said, “That’s what we’re trying to figure out right now … It’s easy to draw conclusions from that, and so we’ve got the shell casings, other forensic evidence that is coming in – and trying to piece all of those things together.”

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Reuters has been unable to locate the roommate or their representatives to seek comment. Reuters could not determine who is serving as Robinson’s legal representative.

According to the UK’s The Sun newspaper, a neighbour of Robinson, Josh Kemp, claimed to have seen the alleged assassin walking hand-in-hand with another person two weeks before Kirk’s killing.

Kemp added that Robinson was wearing all black and that he thought the 22-year-old was “weird”, The Sun reported.

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 US 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-LGBT and anti-immigrant comments, attracted legions of conservatives, but also engendered strong feelings from liberals and drew widespread criticism.

Investigators search for motive

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 420 kilometres south-west 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.

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According to The New York Times, Robinson allegedly responded to messages on Discord posted by friends suggesting that he resembled the suspect by saying that his “doppelganger” was trying to “get me in trouble”.

“Tyler killed Charlie!!!!” another user replied, the Times said.

While Robinson was raised by religious parents in a deeply conservative region of the state, “his ideology was very different than his family”, Cox told NBC on Sunday, 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.

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 a kindred spirit to cross the line into violence – even as the president and his allies have often invoked violent imagery against their opponents.

Mike Johnson, the Republican Speaker of the US House of Representatives, urged calm on Sunday.

“We’ve got to turn the rhetoric down,” Johnson said on the Fox News Sunday program.

In conversations he has had with Republican and Democratic House members since Kirk’s killing, Johnson said, “There’s this recognition that people have got to stop framing simple policy disagreements in terms of existential threats to our democracy.”

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.