The messages that prosecutors say were exchanged between Tyler Robinson and Lance Twiggs on the day of Charlie Kirk’s assassination last year began with goofy expressions of love.

The tone of the exchanges between the couple abruptly shifted when Mr. Robinson said that he was the person who had fatally shot Mr. Kirk, a conservative activist, during an event at Utah Valley University, a four-hour drive from the apartment complex in southern Utah where Mr. Robinson and Mr. Twiggs lived together.

“you werent the one who did it right????” Mr. Twiggs asked Mr. Robinson.

“I am, I’m sorry,” Mr. Robinson replied.

The text messages between the men, along with a handwritten note Mr. Robinson left that had not previously been made public in full, offered the most illuminating view yet into their relationship and what the authorities say were Mr. Robinson’s state of mind and actions in the hours before and after the shooting.

The evidence suggests that he had spent at least several days planning the killing, and that he tried to cover his tracks by retrieving a rifle he stashed in a wooded area and asking Mr. Twiggs to delete their text exchanges.

The messages were laid out in court on Thursday, the fourth day of a preliminary hearing to determine whether there is enough evidence to try Mr. Robinson for aggravated murder. The law-enforcement authorities had previously disclosed many of the messages in earlier filings.

Prosecutors say Mr. Robinson, 23, targeted Mr. Kirk because of the prominent activist’s political beliefs, and the state is seeking the death penalty; Mr. Robinson has yet to enter a plea.

His lawyers sought, unsuccessfully, to preclude the video interview and texts from being presented in this hearing, though they did not specifically contest the authenticity of the messages. The defense had sought to question Mr. Twiggs during the preliminary hearing, but the judge allowed his testimony solely through a prerecorded video.

In the video interview with prosecutors that was played on Thursday, Mr. Twiggs said that Mr. Robinson had left their home in St. George, Utah, at about 4 a.m. on Sept. 10, the day of the shooting, saying he had a long day of electrician work.

At 4:30 p.m., roughly three hours after Mr. Kirk was fatally shot and as a massive manhunt was underway, Mr. Robinson texted Mr. Twiggs: “I love you! Mwah mwah,” giving no indication that anything was amiss.

“I love you moresies mwah mwah mway,” Mr. Twiggs replied.

At 11 that night, Mr. Robinson sent an apparently scheduled text message to Mr. Twiggs to “drop what you are doing” and look under Mr. Robinson’s keyboard, where he had left a note disclosing his plans to kill Mr. Kirk, according to prosecutors’ account of the evidence recovered.

“Luna, If you are reading this per my text, then I am so sorry,” the note read, referring to Mr. Twiggs by a name he sometimes used at the time. “I left the house this morning on a mission and set an auto text. I am likely dead, or facing a lengthy prison sentence.” It continued: “I had the opportunity to take out Charlie Kirk, and I took it.”

Robert Draper contributed reporting.