Tyler Robinson, 22, was arrested on Thursday night after relatives and a family friend alerted authorities that he had implicated himself in the crime, Governor Spencer Cox said on Friday, opening a press conference with the words "we got him".
The arrest capped a 33-hour manhunt for the lone suspect in Wednesday's killing, which President Donald Trump has called a "heinous assassination".
Kirk, co-founder of the conservative student group Turning Point USA and a staunch Trump ally, was gunned down by a single rifle shot fired from a rooftop during an outdoor event attended by 3000 people at Utah Valley University in Orem, about 65 km south of Salt Lake City.
The sniper made his getaway in the ensuing pandemonium, captured in graphic detail in video clips that circulated widely on the internet and television news reports.
A bolt-action rifle believed to be the murder weapon was found nearby, and police released images from surveillance cameras showing a "person of interest" wearing dark clothing and sunglasses.
A break in the case came when a relative and a family friend alerted the local sheriff's office that he had "confessed to them or implied that he had committed" the murder, Cox said.
"I want to thank the family members of Tyler Robinson, who did the right thing in this case and were able to bring him into law enforcement," the governor said.
Security camera footage and evidence gathered from the suspect's profile on the chat and streaming platform Discord also helped investigators link him to the crime, Cox said.
Robinson, a third-year student in the electrical apprenticeship program at Dixie Technical College, part of Utah's public university system, was taken into custody at his parents' house, about 420km southwest of the crime scene.
Investigators on Friday evening collected additional forensic evidence from Robinson's apartment in St George, near the Arizona border.
He was held on suspicion of aggravated murder and other charges that were expected to be formally filed in court early next week, the governor said.
The killing has stirred outrage among Kirk's supporters and condemnation of political violence from across the ideological spectrum.
"It is an attack on all of us," Governor Cox said, calling Kirk's murder a "watershed in American history" and comparing it to the rash of US political assassinations of the 1960s.
Cox declined to discuss possible motives for the killing. But in describing inscriptions investigators found on ammunition recovered from the scene, he said one of the casings bore the message: "Here fascist! CATCH!"
"I think that speaks for itself," he said in response to reporters' questions.
State records show Robinson was a registered voter but not affiliated with any political party. But a relative told investigators Robinson had grown more political in recent years and had once discussed with another family member their dislike for Kirk and his viewpoints, according to an arrest warrant affidavit.
Many Republicans, including Trump, have been quick to lash out at the political left, accusing liberals of fomenting anti-conservative vitriol that would encourage a kindred spirit to cross the line into violence.
Rachel Kleinfeld, a senior fellow at the Carnegie Endowment for International Peace, said the symbology found on the bullet casings suggests the shooter was part of the so-called Groyper movement, associated with far-right activist and commentator Nick Fuentes.
"It's an eclectic ideological movement marked by video game memes, anti-gay, Nick Fuentes white supremacy, irony," she said.Â
"It certainly leans right, but it is quite eclectic."
In her first public comments since her spouse was slain, Erika Kirk vowed in a tearful but defiant video message on Friday evening that "the movement built by my husband will not die" but grow stronger.
Speaking from the studio of his radio-podcast show, she urged young people to join Turning Point, exalting her husband as a fallen political hero who "now and for all eternity will stand at his savior's side wearing the glorious crown of a martyr."
