Charlie Kirk suspected shooter left note saying 'I have the opportunity to take out' conservative activist and will 'take it', FBI boss says

Mr Robinson, 22, remains in custody and is set to be formally charged with the murder tomorrow, Tuesday.

According to Mr Patel, the note was written before the shooting, having been found "in the suspect's home."

Mr Patel said: "We have since learned the note, even though it was destroyed, we have found forensic evidence of the note and we have confirmed what the note said because our aggressive interview posture at the FBI."

Charlie Kirk was killed by a single rifle shot during the event attended by 3,000 people in Orem, about 40 miles south of Salt Lake City.

Utah Governor Spencer Cox said Mr Robinson "is not cooperating, but all the people around him were cooperating, and I think that's very important."

One person who is apparently talking to investigators is Mr Robinson's roommate, who was also a romantic partner, Governor Cox said, citing the FBI.

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Governor Cox described the roommate as "a male transitioning to female," adding they been "incredibly cooperative" to the investigation.

Asked by CNN whether the roommate's gender identity is relevant to the investigation, Governor Cox said: "That's what we're trying to figure out right now.

"It's easy to draw conclusions from that, and so we've got the shell casings, other forensic evidence that is coming in - and trying to piece all of those things together."

Investigators found messages engraved into four bullet casings, which included references to memes and video game in-jokes

One of the inscriptions, according to the affidavit, read: "hey fascist! CATCH!" followed by a combination of directional arrows, an apparent reference to a sequence of button presses that unleashes a bomb in a popular video game.

Another casing, according to the affidavit, read, "If you read This, you are GAY Lmao" with another reading "notices bulge, OwO whats this."

While Mr Robinson was raised by religious parents in a deeply conservative region of the state, "his ideology was very different than his family," Governor Cox told NBC News, without clarifying further.

State records show Mr Robinson was a registered voter but not affiliated with any political party.

A relative told investigators that Mr Robinson had grown more political in recent years and had once discussed with another family member their dislike for Mr Kirk and his viewpoints, according to the arrest warrant affidavit.

The death of Mr Kirk has stirred outrage among his supporters and condemnation of political violence across the ideological spectrum.

President Donald Trump has blamed "the radical left" for Mr Kirk's death.

Pennsylvania Governor Josh Shapiro, a high-profile Democrat who faced an arson attack in April, said President Trump had an obligation to lower the temperature.

"Violence transcends party lines, and the way to address it and have true peaceful debate is for leaders to speak and act with moral clarity. That needs to start with the President," Governor Shapiro said on social media.