The breaking news was announced by US President Donald Trump on a morning television show.
“I think with a high degree of certainty, we have him,” said Trump on the sofa of Fox & Friends on Friday morning in New York City. “In custody.”
“Essentially, someone that was very close to him turned him in.”
It was Trump, too, who first announced that his ally, Kirk, had died after he was shot in the neck at Utah Valley University on Wednesday.
At a press conference on Friday morning, officials identified the person in custody as 22-year-old Tyler Robinson.
Utah Governor Spencer Cox told reporters that “a family member of Tyler Robinson reached out to a family friend who contacted the Washington County Sheriff’s Office with information that Robinson had confessed to them”.
Surveillance video shows the suspect arriving on campus at the university in a Grey dodge challenger at 08:29 local time on the morning of the shooting, said Cox.
He said investigators had interviewed a family member who said the suspect had become more political in recent years.
Cox said: “The family member referenced a recent incident in which Robinson came to dinner prior to September 10, and in the conversation with another family member, Robinson mentioned Charlie Kirk was coming to UVU.”
Cox said investigators had also spoken to a roommate of the suspect who had shown them messages with an account named “Tyler” on the messaging app Discord.
The messages referred to a need to retrieve a rifle from “a drop point” and the rifle being left in a bush, wrapped in a towel.
The FBI said on Thursday they had found the suspected murder weapon – an imported Mauser .30-06 bolt action rifle – wrapped in a towel in a wooded area.
Cox told reporters that investigators had found inscriptions engraved on casings recovered with the rifle, which had a scope mounted on top of it.
The inscriptions included “hey fascist! catch!” and “bella ciao” and “if you read this, you are gay, LMAO”.
Bella ciao means “goodbye beautiful” in Italian. It is also the title of a song dedicated to the Italian resistance who fought against the occupying troops of Nazi Germany.
The arrest came less than a day after the FBI released grainy pictures of a “person of interest” wanted for the shooting.
Investigators appealed for the public’s help identifying the suspect, who was wearing sunglasses, Converse shoes and a “distinctive” long-sleeved black top featuring an American flag and an eagle.
FBI
