The day after Charlie Kirk was assassinated, Tyler Robinson was pacing his Utah apartment, his former partner told investigators.

The partner, Lance Twiggs, had met Mr. Robinson two years earlier as a new roommate who shared some of the same friends. They bonded over video games, grew closer and began dating within months.

Now, Mr. Twiggs told investigators, he confronted Mr. Robinson.

Was Mr. Robinson telling the truth when he had confessed to shooting Mr. Kirk? It was true, Mr. Robinson told Mr. Twiggs, according to an interview with investigators that was played in court Thursday.

“He started crying a little bit,” Mr. Twiggs said. “And said he wishes he hadn’t done it.”

The April interview was the centerpiece of a trove of evidence that prosecutors disclosed in a Provo, Utah, court on Thursday during a hearing to determine whether Mr. Robinson, 23, will face trial. He has been charged with murder and could face the death penalty over the Sept. 10 killing of Mr. Kirk, a conservative activist and ally of President Trump.

The confession to Mr. Twiggs was just one of several Mr. Robinson made, prosecutors said. They presented text messages, police photographs and a handwritten note that offered deeper understanding of his actions and state of mind in the days surrounding the shooting at Utah Valley University in Orem, where Mr. Kirk was appearing before a crowd of students.

Mr. Robinson has not entered a plea, and the judge presiding over the hearing will decide whether prosecutors have met the legal burden to try him. The hearing has unfolded over the past four days as a sort of trial in miniature.

The evidence detailed how, on the day of the killing, Mr. Robinson had left the apartment he shared with Mr. Twiggs in southern Utah “on a mission,” as he described it. Later, prosecutors said, he began to admit his role to friends and his parents before surrendering.

Prosecutors showed messages that they said Mr. Robinson had sent to friends on the Discord app. In them, he confessed but was met with general incredulity. So Mr. Robinson tried to convince his friends, prosecutors said.

“Look at the photos from the surveillance footage,” he wrote. “It was me. Thanks again for everything.”

In the video interview, Mr. Twiggs said Mr. Robinson had left earlier than usual on the day of the shooting, at 4 a.m. or 5 a.m., saying that he had a long trip to his job as an electrician. Mr. Twiggs said he did not hear from him again until late that night, when he got a text instructing him to look at a note under Mr. Robinson’s keyboard.

Mr. Robinson apologized in the note, according to a photograph of it shown publicly for the first time in court. “I’m likely dead or facing a lengthy prison sentence,” the note read.

“I had the opportunity to take out Charlie Kirk, and I took it,” it continued. “I wish we could have lived in a world where this was not necessary.”

As Mr. Twiggs’s interview was played, Mr. Robinson, sitting with his lawyers, had no visible reaction. Mr. Robinson’s father and mother were in the gallery, and his father wore a grave expression.

Significant portions of Mr. Twiggs’s video interview were ordered withheld by Judge Tony Graf, who has said he is trying to balance transparency with Mr. Robinson’s right to a fair trial. A lawyer for Mr. Kirk’s widow and parents has opposed the redaction of any submitted evidence.

Mr. Twiggs told investigators that he and Mr. Robinson had met in 2023, but not until after the shooting did he hear Mr. Robinson mention Mr. Kirk. In a text exchange released by prosecutors, Mr. Robinson said that he had “had enough” of Mr. Kirk, adding, “Some hate can’t be negotiated out.”

Prosecutors said last year that Mr. Twiggs had been transitioning to identify as a woman, but both prosecutors and Mr. Robinson’s lawyers have this week referred to him as a man. Mr. Twiggs said in the interview that, for a time, he went by “Luna” to some friends, which is how Mr. Robinson addressed him in the note.

Mr. Robinson’s mother told the police that her son had moved to the left and had become “more pro-gay and trans-rights oriented” in the period before the shooting.

Prosecutors say Mr. Robinson targeted Mr. Kirk because of his views. Mr. Kirk had said gender transitions conflict with Christian teachings, calling them a “middle finger to God.”

Mr. Twiggs said that Mr. Robinson occasionally brought up politics, but that the two had talked little about gender identity or gay or transgender rights.

When investigators interviewed Mr. Twiggs, they showed him several images from surveillance cameras on the university campus that captured the man they believed to be the assassin.

As they scrolled through the images, Mr. Twiggs noted the person’s shoes, sunglasses, hat and jeans, and said the person looked like Mr. Robinson. Then he was shown more images, and grew confident.

“The last two definitely do look like him,” he said. “They do look like Tyler Robinson.”