Tyler Robinson appeared quiet and sombre when he arrived with his parents to turn himself in last week at the Washington County Sheriff's office, said Sheriff Nate Brooksby.
It was a day after Kirk was shot and killed at Utah Valley University on September 10.
"He didn't want a big SWAT team at his parents' house or his apartment," said the sheriff, who was only involved with the surrender and not the broader investigation.
"He was truly fearful about being shot by law enforcement."
Prosecutors on Tuesday charged the 22-year-old Robinson with capital murder and announced they will seek the death penalty.
They revealed a series of incriminating messages and DNA evidence they say connect Robinson to the killing of Kirk, a prominent conservative activist and confidant of US President Donald Trump.
Robinson also faced his first hearing in the case on Tuesday where a judge said he would appoint a lawyer to represent him.
The shooting stunned the nation and exposed deep political divides. The president has threatened to crack down on what he calls the "radical left" and classify some groups as domestic terrorists.
Trump said early on Thursday US time he was "designating" the anti-fascist Antifa movement as a terrorist organisation but gave no detail on the plan.
"I will also be strongly recommending that those funding ANTIFA be thoroughly investigated in accordance with the highest legal standards and practices," Trump wrote on his Truth Social platform.
Former president Barack Obama said this week Trump has further divided the country rather than work to bring people together.
American broadcaster ABC suspended Jimmy Kimmel's late-night show indefinitely following comments he made about the reaction to Kirk's killing.
The shooting at Utah Valley happened more than a three-hour drive from where Robinson grew up.
That's why the sheriff in Washington County said he was stunned when a retired detective and a friend of the Robinson family called to say he knew the shooter's identity and they were trying to convince him to come in voluntarily.
"I couldn't fathom what actually came out of his mouth," Brooksby said.
The sheriff said he was told Robinson was possibly suicidal and in a remote area of southwest Utah.
An hour later, Robinson arrived at the sheriff's office where he was greeted by plainclothes detectives.
"He knew it was inevitable that he would be caught," said the sheriff. "If at the end of the day we accomplish him surrendering peacefully on his own, I'm going to make some concessions to make that happen."
Investigators say after Robinson fired the single fatal shot, he texted his romantic partner and said to look under a keyboard for a note.
"I had the opportunity to take out Charlie Kirk and I'm going to take it," it said, according to court documents.
After expressing shock, his partner, who lived with Robinson, asked if he was the shooter. Robinson responded, "I am, I'm sorry."
Utah County Attorney Jeff Gray said DNA on the trigger of the rifle used to kill Kirk matched Robinson.
Law enforcement officials say they are looking at whether others knew about Robinson's plans or helped, but they have not said if his partner is among those being investigated, only expressing appreciation for the partner sharing information.
