Kirk, who brought young, conservative evangelical Christians into politics, was shot on Wednesday while speaking at Utah Valley University during one of his many campus stops.
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DNA on a towel wrapped around a rifle found near where conservative activist Charlie Kirk was assassinated matches that of the 22-year-old accused of his killing, FBI Director Kash Patel said on Monday.
Investigators have also used DNA evidence to link the suspect, Tyler Robinson, with a screwdriver recovered from the rooftop where the fatal shot was fired on 10 September, Patel told Fox News Channel on Monday.
Authorities in Utah are preparing to file capital murder charges against Robinson as early as Tuesday in the killing of Kirk, a dominant figure in conservative politics who became an ally of President Donald Trump after founding the Arizona-based "Turning Point USA," one of the country's largest political organisations.
Kirk, who brought young, conservative evangelical Christians into politics, was shot on Wednesday while speaking at Utah Valley University during one of his many campus stops.
The shooting raised fears about increasing political violence in a deeply polarised United States.
Officials have said Robinson had a hatred for Kirk and was an advocate of "leftist ideology" that had grown in recent years.
Robinson's family and friends said he spent large amounts of time scrolling the "dark corners of the internet," Utah Governor Spencer Cox said on Sunday.
Patel told Fox News that Robinson had written in a note before the shooting that he had an opportunity to take out Kirk and was going to do it.
Investigators were able to recover the note's contents after it had been destroyed, the FBI director said, paraphrasing from the note without revealing more details.
Authorities said Robinson has not been cooperating with law enforcement. They say that he may have been "radicalised" online and that ammunition found in the gun used to kill Kirk included anti-fascist and meme-culture language engravings.
Court records show that one bullet casing had the message, "Hey, fascist! Catch!"
Robinson was arrested late on Thursday near where he grew up around St. George, in the southwestern corner of Utah between Las Vegas and Bryce Canyon and Zion National Parks.
It's unclear at this stage whether he has an attorney who could speak on his behalf.
Tributes to Kirk continue to pour in across the United States. A line of mourners attended a vigil at the Kennedy Centre in Washington on Sunday and there were moments of silence at several professional sporting events.
Vice President JD Vance, who counted Kirk as a close friend, said he planned to serve as a substitute host for Kirk's talk show on the Rumble streaming platform.
"Please join me as I pay tribute to my friend," Vance wrote in a social media post.