Here’s what to expect from the charges and first hearing.

Here’s what we’re covering:

• Suspect’s texts: Before he was arrested, Mr. Robinson sent text messages to friends on the chat app Discord indicating that he was closely following news about the killing, and joked that his “doppelganger” had done it. In another chat, which the app said it had shared with law enforcement officials, Mr. Robinson reportedly told friends, “It was me.” Read more ›

• Written note: Kash Patel, the F.B.I. director, said Mr. Robinson had also written a note saying that he was going to make use of an opportunity to kill Mr. Kirk. Mr. Patel, who was testifying before Congress on Tuesday morning, has faced scrutiny over his leadership of the agency and the Kirk investigation. Read more ›

• Seeking motive: Gov. Spencer Cox, Republican of Utah, said in a television interview on Sunday that Mr. Robinson held a “leftist ideology,” and had a romantic partner who was transitioning from male to female. Messages left on bullet casings linked to the shooting, however, are less clear about a motive. Read more ›

• National fallout: Trump administration officials have threatened to crack down on what they say is a left-wing network that funds and incites violence against conservatives — without presenting evidence that such a network exists. Public figures associated with both parties have faced violence, including fatal attacks, in recent years. Read more ›

The message was sent Thursday, two hours before Robinson surrendered to the authorities, Discord said. A spokeswoman said an internal investigation found the account that belonged to Mr. Robinson, including the message, but added that there was “no evidence that the suspect planned this incident or promoted violence on Discord.”

Discord is a social network popular with the video gaming community, made up of many public and private chat rooms. The group that Robinson wrote to included about 30 people, including some of his online friends.

Here is what to expect:

• The Utah County attorney, Jeff Gray, said he plans to file formal charges by noon local time on Tuesday. There have been no indications so far that Mr. Robinson will face federal charges, though federal officials have been closely involved in the investigation.

• Shortly after the charges are filed, Mr. Gray plans to hold a news conference to explain them and the next steps. If convicted of aggravated murder in Utah, defendants can face the death penalty, life in prison, or 25 years to life in prison with the possibility of parole.

• Later on Tuesday, Mr. Robinson is expected to make his first court appearance via video conference from the jail. In a brief hearing, the charges will be read to him, and the judge will appoint a public defender if he does not have a lawyer.

Speaking on the “Fox & Friends” television show Monday morning, Mr. Patel said that Mr. Robinson had suggested before the shooting that he was going to kill Mr. Kirk, in a text message exchange and in a written note. He said that the note was destroyed but that the authorities were able to reconstruct it.

It was unclear to whom the text message was sent. Mr. Patel did not say how the physical note had been destroyed or if anyone other than the suspect knew about it or had seen it before the shooting.

The killing of Mr. Kirk, 31, a right-wing activist, stunned the nation last week, when he was struck by a single gunshot while speaking to an audience of students at Utah Valley University in Orem, Utah. The authorities began an intensive manhunt, and Mr. Robinson turned himself in about 30 hours after the shooting at a sheriff’s office near his home in southwestern Utah, more than 250 miles from the campus.

The motivation for the shooting has been fiercely debated, with investigators and civilians alike scouring the suspect’s online presence and trying to gather information from his friends and relatives.

Gov. Spencer Cox of Utah, a Republican, and Mr. Patel have said people who know Mr. Robinson have told investigators that he subscribed to what the two men called left-wing or leftist ideology.

Mr. Cox has said that the suspect was a normal young man who became “radicalized” after dropping out of college, and that his family was conservative.

The governor also said that a person who was initially described as a roommate of Mr. Robinson’s was actually his romantic partner, and that the person was transitioning from being a man to being a woman. That person, the governor said, has been cooperating with the investigation and was “shocked” by the killing.

Friends have described Mr. Robinson as a very smart, if somewhat quiet, person who enjoyed playing video games and took an interest in current events. He was registered to vote but unaffiliated with any political party, and he had no previous criminal history.

President Trump said in the Oval Office on Monday that he believed the suspect may have been “radicalized online.”

“He went bad, and he just went bad very quickly,” Mr. Trump said. “He sort of led a very normal life, a life of great education, schools, everything else, but somewhere along the line something happened.”

Messages that the suspect sent after the shooting, which were obtained by The New York Times, showed that he joked with friends about the fact that he resembled the person being sought by the F.B.I. He said in the messages that his “doppelgänger” was trying to “get me in trouble,” and joked that he needed to “get rid of this manifesto and exact copy rifle” that he had lying around.

On Thursday, about nine hours after sending those messages, he turned himself in after his father recognized him in higher quality images released by the authorities.

During the TV appearance on Monday, Mr. Patel promised that no one at the F.B.I. would politicize the investigation. He has faced criticism for posting on social media soon after the shooting that a “subject” had been apprehended; in fact, that person had nothing to do with the shooting and was quickly released.

“Could I have worded it a little better, in the heat of the moment? Sure,” Mr. Patel said on Monday about his post. “But do I regret putting it out? Absolutely not. I was telling the world what the F.B.I. was doing.”

Tagging Mr. Robinson’s username on Discord, a messaging platform, the acquaintance attached the images and wrote “wya” — where you at? — with a skull emoji, suggesting that Mr. Robinson, 22, looked like the man being sought.

Mr. Robinson replied within a minute. His “doppelganger,” he wrote, was trying to “get me in trouble.”

“Tyler killed Charlie!!!!” another user wrote, apparently in jest.

That was on Thursday afternoon, around 1 p.m. local time. It was not until later that night, nearly 34 hours after the shooting, that Mr. Robinson was arrested on suspicion of carrying out the assassination of Mr. Kirk, 31, a conservative pundit whose killing has inflamed the political world.

The Discord messages were the clearest glimpse yet of the suspect’s demeanor in the hours after the killing. They were shared with The New York Times by someone who knew Mr. Robinson in high school and has kept up with him in the group chat — which includes about 20 people — but said he had not seen Mr. Robinson in person for several years. He spoke on the condition that his name not be used, fearing harassment for being an acquaintance of the suspect.

The Times independently verified that the person who shared the screenshots attended high school with Mr. Robinson, and found other indications that they knew one another. The screenshots were shared at the request of The Times. Discord declined to confirm Mr. Robinson’s username, but it matches several other accounts that he used elsewhere online.

The suspect is expected to be formally charged by local prosecutors on Tuesday. It was not clear on Saturday if he had a lawyer.

The messages do not shed light on a possible motivation for the shooting, which has been fiercely debated by people trying to ascribe blame to a political side. A police officer wrote in an affidavit that Mr. Robinson had recently discussed Mr. Kirk’s upcoming event in Utah with a family member, and that the two had “talked about why they didn’t like him.”

People who knew Mr. Robinson over the years said that he was extremely intelligent, followed current events, and spent much of his time online or playing video games. He was registered to vote but was not affiliated with a political party and appeared to have never voted in an election; his parents are both registered Republicans.

On Sunday, Gov. Spencer Cox of Utah said in a television interview that Mr. Robinson had a “leftist ideology," but he also cautioned that it could take time for an exact motivation to emerge.

After Mr. Robinson joked that the gunman was his look-alike, another user suggested that the group could turn Mr. Robinson in and get the $100,000 reward that the F.B.I. was offering.

“Only if I get a cut,” Mr. Robinson responded.

Someone posted, “Whatever you do, don’t go to a mcdonalds anytime soon,” a reference to the arrest of Luigi Mangione, who was found at a McDonald’s restaurant and charged with the killing of a health insurance chief executive in Manhattan.

Mr. Robinson agreed and offered a supposed joke of his own, writing “better also get rid of this manifesto and exact copy rifle I have lying around.”

When another user suggested that the killing of Mr. Kirk would lead President Trump to send the National Guard to Utah, the suspect replied, “in a red state??? nah CLEARLY the shooter was from california.”

Around that time, several news outlets reported that ammunition found near a rifle at the scene included engravings referencing “transgender ideology.” The truth, ultimately, was that the engravings included the phrase “hey fascist! CATCH!” as well as lyrics from an antifascist Italian folk song and a reference to a sexual meme about a “bulge.”

But at the time, before the exact phrasing of the engravings was publicly known, the suspect sent messages that suggested he was closely following the news.

“I heard the ammo had somethin about trans stuff on it, but they aren’t releasing photos or exact quotes,” he wrote. He added: “and also the claim wasn’t backed by the official fbi, just some dude in the briefing room.”

Utah’s governor said in the television interview on Sunday that a person previously described as a roommate of the suspect was in fact a boyfriend who was transitioning to become a woman, and he said that person was fully cooperating with authorities and was “shocked” by what had happened.

Mr. Robinson’s messages on Discord appeared next to his avatar, which was from a Garfield comic and depicted the confused face of Garfield’s owner, Jon Arbuckle.

At one point, he joked in a message: “I’m actually Charlie Kirk, wanted to get outta politics so I faked my death, now I can live out my dream life in kansas.”

The police have said that the suspect walked onto campus ahead of the event, in which Mr. Kirk debated students with opposing beliefs, and climbed up to the roof of a building about 400 feet away from where Mr. Kirk was speaking. The gunman fired one shot, they said, fatally striking Mr. Kirk, and then dropped down from the roof and rushed off campus.

The police said that Mr. Robinson had implied or confessed that he committed the crime, and that he was then encouraged by a family member and family friend to turn himself in, which he did. He surrendered to the police in Southwestern Utah, where he lived, more than a three hours’ drive from the campus where the shooting took place.

Following the suspect’s arrest on Thursday night, members of the group chat on Discord struggled to believe that he was actually being accused of the crime. “Our governor wants to give him the death penalty dude,” one wrote.

Another said: “I truly cannot distinguish if this is for real.”

Aric Toler contributed reporting. Sheelagh McNeill contributed research.

If the reference to fascism appeared to be straightforward, the arrows were most likely understandable only to certain subsets of gamers. They seem to refer to the popular video game Helldivers 2 and its sequence of controller moves to unleash a powerful bomb.

“It’s a joke in the Helldivers community that you can shut down any argument you disagree with by entering ^ > vvv and blowing the whole thing up,” Steve Iannelli, 38, a mechanical engineer and experienced player of the game who lives in Baltimore, said in an email.

The messages, which the authorities believe the suspect etched, are some of the few known clues to his possible motivations. Late Thursday, they arrested Tyler Robinson, a 22-year-old Utah man, and accused him of killing Mr. Kirk.

Perpetrators of high-profile violence have often used such messages to communicate their beliefs. And in some of the most violent attacks of the last decade, the young gunmen used the culture of small internet forums to try to both justify their attacks and appeal to the niche groups they believed would approve of their actions.

The messages found on the casings — including puerile jokes and a reference to a popular Italian song — are rooted in that coded communication style of the habitually online.

Even the reference to fascism has a clear echo in Helldivers 2, a satirical science fiction game in which the player battles an alien invasion on behalf of an Earth that is ruled by a thinly veiled fascistic government.

But these messages are difficult to parse. Internet in-jokes and references are slippery things, often deployed with multiple layers of irony. That left many Americans trying to crack the enigmatic messages: Was Mr. Robinson a man of the left or of the right, or something else entirely?

In a news conference on Friday morning, Gov. Spencer Cox of Utah, a Republican, said that the phrase “hey fascist! CATCH!” clearly showed the gunman’s intent. “I think that speaks for itself,” he said.

But even that message, which many on the right believed placed Mr. Robinson on the far left, may not be a reliable signpost to the suspect’s political beliefs.

“It’s very hard to map a political ideology on this mishmash of video game references and hints of different internet subcultures,” said Emerson Brooking, a fellow at the Atlantic Council, an international-affairs think tank, and a former cyberpolicy adviser at the Defense Department.

Perpetrators’ use of language from the online world is not confirmation that violent actors are influenced by video games. The American Psychological Association’s standing position is that there is “insufficient scientific evidence to support a causal link between violent video games and violent behavior.”

But the use of language does say something about the ubiquity of online gaming culture. A recent Pew survey found that 85 percent of U.S. teenagers play video games, with 40 percent considering themselves to be “gamers.”

In some cases, attackers have explained themselves clearly. In 2019, when a gunman opened fire on mosques in Christchurch, New Zealand, killing 51 people and injuring dozens more, he posted a manifesto that cited popular far-right memes. Containing white nationalist rhetoric, Islamophobic slurs and a copypasta — a rant that is copied and pasted multiple times — it was designed to appeal to a fringe community. Both his livestream of the attack and the manifesto were viewed millions of times and continue to be posted to online forums today.

Three years later, in May 2022, when a gunman killed 10 people in Buffalo, N.Y., he, too, posted a screed, describing how he had drawn inspiration from the Christchurch shooting and citing some of the same memes.

As of Friday evening, much about Mr. Robinson and his motives remained unclear. He is registered to vote in Utah, but he is not affiliated with a political party and had never voted in an election, according to the Washington County Clerk.

In an interview on Friday, Adrian Rivera, 22, who had taken a woodworking class with Mr. Robinson, said he was interested in “random nerdy stuff,” calling him a “massive Halo guy” and noting that he liked to play Call of Duty and other shooter games.

Mr. Rivera said he did not know whether Mr. Robinson had political views. According to the affidavit, investigators interviewed one of his family members, who said that Mr. Robinson had “become more political in recent years.”

Through interviews with a relative and a roommate, investigators also learned that the suspect had criticized Mr. Kirk not long before the shooting, Governor Cox said.

The messages on the casings are not much more revealing.

One of the messages, “Notices bulges OwO what’s this?,” is often used to mock participants in online role-play communities. Another message said, “If you read This, you are GAY Lmao,” its tone suggesting a kind of sophomoric insult humor common on internet message boards.

Then there was the message that read, “O Bella ciao, Bella ciao, Bella ciao, Ciao, ciao!,” likely a reference to the popular Italian folk song “Bella Ciao.” Popularized as an antifascist anthem in Italy after World War II, it resurfaced globally in recent years because of its inclusion in the hit Netflix series “Money Heist” and in video games, including the first-person shooter game Far Cry 6.

The song is still well-known as antifascist. It was sung as a protest last year by progressive members of the European Parliament during a visit by Viktor Orban, Hungary’s far-right prime minister.

But a number of people noted online on Friday that a version appears on a Spotify playlist meant for Groypers, the followers of Nick Fuentes, the white nationalist who occupies a political space to the right of, and in opposition to, Mr. Kirk.

As people online debated what the references meant, Mr. Brooking mused that the discussion might amount to a mission accomplished for a troll.

“The spectacle,” he said, “has to be the entire point.”

Sabrina Tavernise and Joe Coscarelli contributed reporting.