A police mugshot shows Tyler Robinson. [Source: Reuters]

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, Utah Governor Spencer Cox said on Sunday.

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

Investigators have 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 3,000 people in Orem, about 40 miles south (65 km) 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.”

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 cooperative.”

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’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,” short for “laughing my ass off.”

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