The man arrested in the killing of right-wing activist Charlie Kirk is not cooperating with authorities, but investigators are working to establish a motive for the shooting by talking to his friends and family, according to Utah Governor Spencer Cox.
Mr Cox said the accused gunman, 22-year-old Tyler Robinson, would be formally charged on Tuesday.
He remains in custody in Utah.
Investigators have yet to piece together why Mr Robinson allegedly scaled a rooftop at Utah Valley University during an outdoor event and shot Mr Kirk in the neck at long range on Wednesday.
Mr 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 3,000 people in Orem, about 65 kilometres south of Salt Lake City.
The killing ushered in new fears of a spike in political violence in the United States and an ever-deepening divide between the left and the right.
Mr Robinson has not confessed to investigators, Mr Cox said.
"He is not cooperating, but all the people around him were cooperating, and I think that's very important."
One person who is apparently talking to investigators is Mr Robinson's roommate, who was also a romantic partner, Mr Cox said, citing the FBI.
The governor described the roommate as "a male transitioning to female", and said the roommate had been "incredibly cooperative".
Reuters has not been able to locate the roommate, or representatives for the roommate, to seek comment.
Asked on CNN's State of the Union program whether the roommate's gender identity was relevant to the investigation, Mr 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."
Investigators found messages engraved into four bullet casings, which included references to memes and video game in-jokes.
An affidavit filed by authorities in the case described these messages. One of the inscriptions, according to the affidavit, 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".
Mr 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
Mr 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, 420km 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.
While Mr Robinson was raised by religious parents in a deeply conservative region of the state, "his ideology was very different than his family", Mr Cox said on Sunday without going into specifics.
State records show the 22-year-old was a registered voter but not affiliated with any political party.
A relative told investigators he had grown more political in recent years and had once discussed with another family member their dislike for Mr Kirk and his viewpoints, according to the arrest warrant affidavit.
Mr Robinson was "not a fan" of Mr Kirk's, Mr Cox said on Sunday.
The killing has stirred outrage among Mr Kirk's supporters and condemnation of political violence from some across the ideological spectrum.
Many Republicans, including Mr Trump, have been quick to lash out at the political left, accusing liberals of instigating 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," he said on Fox News.
In conversations he has had with Republican and Democratic House members since Mr Kirk's killing, Mr Johnson said, "There's this recognition that people have got to stop framing simple policy disagreements in terms of existential threats to our democracy."
But Mr Johnson also criticised Democrats.
"You can't call the other side fascists and enemies of the state and not understand that there are some deranged people in our society who will take that as cues to act and do crazy and dangerous things. And that's what we've seen in increasing frequency," he said.
Meanwhile, the Utah governor assigned some blame to social media, saying it had "played a direct role in every single assassination and assassination attempt that we have seen over the last five, six years".
Mr Trump has credited Mr Kirk with driving young voters to conservatism. His Turning Point movement says it has more than 800 chapters across college campuses.
A memorial event for Mr Kirk will be held on September 21 in Glendale, Arizona.
Reuters
