The FBI is leading a manhunt for the killer of Charlie Kirk, a conservative activist and close ally of US President Donald Trump, who was shot dead at a college event in Utah.

The co-founder and CEO of the youth organisation Turning Point USA, the 31-year-old Kirk is the latest victim in a spasm of political violence across the United States.

Videos posted to social media from Utah Valley University show Kirk speaking into a handheld microphone while sitting under a white tent emblazoned with the slogans"The American Comeback" and "Prove Me Wrong".

A single shot rings out and Kirk can be seen reaching up with his right hand as a large volume of blood gushes from the left side of his neck.

Stunned spectators are heard gasping and screaming before people start to run away.

The AP was able to confirm the videos were taken at Sorensen Centre courtyard on the Utah Valley University campus.

Kirk's organisation, Turning Point USA, has announced his death in a note to employees and supporters.

"It's with a heavy heart that we, the Turning Point USA leadership team, write to notify you that early this afternoon, Charlie went to his eternal reward with Jesus Christ in Heaven."

A Turning Point USA official shared the note with CNN.

A person who was taken into custody at Utah Valley University was not the suspect, according to a person familiar with the investigation who was not authorised to speak publicly.

The mayor of Orem, Utah, David Young, says the shooting suspect remains at large.

Authorities are not telling people to shelter in place.

FBI co-leads manhunt for shooter

The FBI has joined Salt Lake City police in the manhunt for Kirk's assassin.

The Salt Lake City bureau said it has dedicated"full resources" to finding and arresting the shooter.

"As soon as we heard about the shooting, special agents and personnel from the Salt Lake City Field Office responded immediately," Special Agent in Charge Robert Bohls said in a statement.

"We have full resources devoted to this investigation, including tactical, operational, investigative and intelligence."

Bohl said the FBI was "co-leading" the investigation alongside local police.

Map shows shooter's vantage point

Kirk was shot and killed while at an event at Utah Valley University, located in Orem, Utah.

He was talking into a microphone in the Sorensen Centre courtyard, sitting under a white tent, when the shooting took place, according to videos on social media.

The university has said the shooter fired from the Losee Centre, which is located roughly 180 metres from where Kirk was sitting.

It has been reported the shooter may have been on the roof.

The below map shows the exact location of the fatal shooting.

Witness describes terror and panic after shooting

A witness who was watching Kirk speak at Utah Valley University said the shooting turned the event into a"nightmare".

Ryan DeVries, 25, told Associated Press he heard"popping" before seeing Kirk's head slump on stage.

The shooting then triggered mass panic.

"People definitely feared for their lives. I could see it in their eyes. I could hear it in their voices," DeVries said.

"People were crying. People were screaming. It was a nightmare."

DeVries said he was also surprised by the lack of security at the event

Kirk was speaking at a debate hosted by his nonprofit political organisation. The event had been met with divided opinions on campus.

An online petition calling for university administrators to bar Kirk from appearing received nearly 1000 signatures.

The university issued a statement last week citing First Amendment rights and affirming its"commitment to free speech, intellectual inquiry, and constructive dialogue."

Last week, Kirk posted on X images of news clips showing his visit to Utah colleges was sparking controversy.

He wrote,"What's going on in Utah?"

The college said a "single shot" had been fired at Kirk and a suspect was in custody.

Police later named two people who were arrested and released following Kirk's assasination.

Utah Police said George Zinn was initially arrested as a suspect but has since been released after being charged with obstruction by UVU police.

A second suspect, Zachariah Qureshi, was also taken into custody and released after an interrogation.

Trump leads tributes

Trump paid tribute to Kirk on social media.

"The Great, and even Legendary, Charlie Kirk, is dead," he said.

"No one understood or had the Heart of the Youth in the United States of America better than Charlie.

"He was loved and admired by ALL, especially me, and now, he is no longer with us. Melania and my Sympathies go out to his beautiful wife Erika, and family. Charlie, we love you!"

He also ordered all US flags be lowered to half mast in Kirk's honour.

"In honor of Charlie Kirk, a truly Great American Patriot, I am ordering all American Flags throughout the United States lowered to Half Mast until Sunday evening at 6 P.M," Trump wrote in a following Truth Social post.

A host of Republican and Democratic elected officials earlier decried the shooting and offered prayers for Kirk on social media.

Utah Governor Spencer Cox said he spoke to Trump following Kirk's death.

"I just got off the phone with President Trump. Working with the FBI and Utah law enforcement, we will bring to justice the individual responsible for this tragedy," Cox wrote on X.

"Abby and I are heartbroken. We are praying for Charlie's wife, daughter, and son," Cox added.

The shooting comes amid a spike in political violence in the US across all parts of the ideological spectrum.

The attacks include the assassination of a Minnesota state lawmaker and her husband at their house in June, the firebombing of a Colorado parade to demand Hamas release hostages, and a fire set at the house of Pennsylvania's governor, who is Jewish, in April.

The most notorious of these events is the shooting of Trump during a campaign rally last year.

Former Utah congressman Jason Chaffetz, a Republican who was at the event, said in an interview on Fox News Channel that he heard one shot and saw Kirk go back.

"It seemed like it was a close shot," Chaffetz said, who seemed shaken as he spoke.

He said there was a light police presence at the event and Kirk had some security but not enough.

"Utah is one of the safest places on the planet," he said.

"And so we just don't have these types of things."

Turning Point was founded in suburban Chicago in 2012 by Kirk, then 18, and William Montgomery, a Tea Party activist, to proselytise on college campuses for low taxes and limited government. It was not an immediate success.

But Kirk's zeal for confronting liberals in academia eventually won over an influential set of conservative financiers.

Despite early misgivings, Turning Point enthusiastically backed Trump after he clinched the GOP nomination in 2016. Kirk served as a personal aide to Donald Trump Jr, the president's eldest son, during the general election campaign.