The arrest of 22 year old Tyler Robinson, marks a major break in a case that shocked the country and raised fresh alarms about political violence in a deeply polarised United States.
Utah's Republican Governor Spencer Cox credited Robinson's family with helping turn him over to authorities.
Robinson is believed to have acted alone, and the investigation is ongoing, Cox said. He was arrested on suspicion of capital murder, weapons and obstruction offences. He was expected to be formally charged on Tuesday ahead of an initial court appearance.
Robinson's arrest capped a frenetic day-and-a-half search that had authorities pleading for tips and leads from the public.
The assassination while Kirk was speaking at Utah Valley University reverberated across the country because of his outsize influence in conservative circles, his close connections with Trump and because of the questions it raised about the escalating toll of political violence that has spanned the ideological spectrum.
"This is our moment: Do we escalate or do we find an off-ramp?" Cox asked, making an impassioned plea for young people to bridge differences through common ground rather than violence. "It's a choice."
Authorities described evidence they said shed potential light on the slaying.
That included the engravings on bullet casings, including one that said, "Hey, fascist! Catch!" Cox said.
A roommate had shared with authorities messages from the chatting app Discord that involved a contact named Tyler and discussed a rifle wrapped in a towel, engraved bullets and a scope, the governor said. A Mauser .30-calibre, bolt-action rifle was found in a towel in a wooded area along the path investigators believe Robinson took after firing a single shot from a distant roof and then fleeing.
The clothes the suspect wore when confronted by law enforcement matched what he had on when he arrived on campus, and a family member confirmed he drove a grey Dodge Challenger like the one seen in a surveillance video, Cox said.
Robinson's father recognised him from the photos released by the FBI and told him to turn himself in. Robinson refused at first, but then changed his mind, according to an official. His father contacted their youth pastor, who also occasionally works with the US Marshals and called the agency so Robinson could turn himself in.
Meanwhile, investigators continued to dig into the background of Robinson, who was admitted to Utah State University, about 80 miles north of Salt Lake City, on a prestigious academic scholarship. However, he attended for only one semester in 2021, a spokesperson said.
Robinson currently is enrolled in an electrical apprenticeship program at Dixie Technical College near his hometown, in southern Utah.
At his family's home, window coverings were drawn, and a pickup truck blocked access to the driveway. There was a heavy law enforcement presence, with several police vehicles parked along the street, which was closed at both ends.
Kirk was a conservative provocateur who became a powerful political force by rallying young Republican voters and was a fixture on college campuses, where he invited sometimes-vehement debate on social issues.
He was taking questions from an audience member about gun violence when a shot rang out. Kirk reached up with his right hand as blood gushed from the left side of his neck.
Trump, who was joined by Democrats in condemning the violence, said he would award Kirk the Presidential Medal of Freedom, the nation's highest civilian honour.
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