US authorities have arrested a man who they say confessed or "implied" to a family friend that he killed right-wing activist Charlie Kirk.
The suspect has been identified as Tyler Robinson, a 22-year-old man from Utah.
"We got him," Utah Governor Spencer Cox told a press conference.
Mr Robinson was taken into custody on Thursday night, local time, about 33 hours after Mr Kirk was shot while speaking at a Utah university, FBI Director Kash Patel said.
Official news of the arrest came shortly after US President Donald Trump used a TV appearance to say he had "a high degree of certainty" the killer was in custody.
"I hope he gets the death penalty," Mr Trump told Fox News.
Authorities later said messages were inscribed in bullet casings that were found with the rifle believed to be the murder weapon.
One of the inscriptions said: "Hey fascist, catch!"
Another said: "If you read This, you are GAY Lmao".
A third said: "Oh, Bella Ciao, Bella Ciao, Bella Ciao, Ciao," a likely reference to an anti-fascist Italian folk song.
Mr Kirk's widow, Erika Kirk, made her first public comments since his death in a live broadcast on the social media of Turning Point USA, Mr Kirk's organisation.
"I want to thank the millions of people who have shown their love for Charlie here in Phoenix, across America, and worldwide," she said in the message on Friday night, local time.
"Charlie loved life. He loved his life, he loved America, he loved nature, which helped him always bring him closer to God.
"Already I have seen the worldwide outpouring of love for my husband … my husband's mission will not end, not even for a moment."
Father 'convinced' suspect to give himself up
Mr Trump said he had been told a minister and the suspect's father had been involved in turning him in.
"The father convinced the son: 'This is it,'" Mr Trump said.
Mr Robinson's father recognised him from CCTV images released by the FBI and encouraged his son to turn himself in, a law enforcement source told the Associated Press.
His father then contacted a youth pastor, who helped Mr Robinson turn himself in.
Mr Cox told reporters Mr Robinson had contacted a family friend and confessed or "implied that he had committed the incident".
Mr Robinson has not yet been formally charged. But an affidavit shows he was arrested for aggravated murder, felony discharge of a firearm causing serious injury, and obstruction of justice, according to the Reuters news agency.
Utah County Attorney Jeff Gray plans to file formal charges next Tuesday, his office confirmed to the ABC.
Relatives recount conversations for investigators
A family member told authorities Mr Robinson had become more political in recent years and had spoken in a disparaging manner about Mr Kirk, Mr Cox said.
"The family member referenced a recent incident in which Robinson came to dinner prior to September 10," he said.
"In the conversation with another family member, Mr Robinson mentioned Charlie Kirk was coming to UVU [Utah Valley University].
"The family member also stated Kirk was full of hate and spreading hate."
The affidavit, filed with a court in Utah, also says investigators have spoken to Mr Robinson's current or former housemate.
The housemate showed investigators messages in which Mr Robinson talked about retrieving a rifle from a drop point, then ditching it in a bush wrapped in a towel, the document says.
Social media posts punished
Authorities have meanwhile moved to punish government agents and public service workers who have expressed negative opinions about Mr Kirk online.
A Secret Service agent was placed on leave and had his security clearance revoked after writing on Facebook that Mr Kirk "spewed hate and racism on his show", according to Fox News.
A Florida teacher was suspended and three more were being investigated over comments they made about Mr Kirk on their personal social media accounts, Florida public radio reported.
The suspected murder weapon, a bolt-action rifle, was found on Thursday in an area where the killer was believed to have fled.
Mr Kirk was shot while speaking at the university a day earlier, in what authorities called a political assassination.
He was on the campus as part of a national tour of universities to hold public debates with progressive students.
Video taken just after the shooting showed a person running across the roof of a campus building.
Mr Kirk's shooting follows a number of apparently politically motivated attacks in the US in recent years, including apparent assassination attempts on Mr Trump last year.
They also include the shootings of two Democratic Minnesota politicians and their spouses in June, an arson attack on the home of Democratic Pennsylvania Governor Josh Shapiro in April, and the 2022 hammer attack on the husband of Democratic former house speaker Nancy Pelosi.
