Investigators released two images of the person and appealled for tips to help solve the shooting of Charlie Kirk, a conservative activist and close ally of President Donald Trump who played an influential role in rallying young Republican voters.
The unidentified suspect is believed to have jumped off a roof and fled into a neighbourhood after firing one shot, authorities said on Thursday.
A high-powered, bolt-action rifle they believe was used in the attack was recovered, and video recordings of the person they believe was responsible are being reviewed.
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The FBI says it's offering a reward of up to $US100,000 ($150,000) for information leading to the identification and arrest of whoever is responsible for Kirk's death.
Kirk's assassination renews attention to escalating threats of political violence in the United States that cut across the ideological spectrum.
His killing is drawing bipartisan condemnation, but a national reckoning over ways to prevent political grievances from manifesting as deadly violence appears elusive.
The shooter appeared to be of "college age" and is believed to have blended in on the university campus where Kirk was shot, authorities said as they investigated the latest act of political violence to befall America.
"I can tell you this was a targeted event," said Robert Bohls, the top FBI agent in Salt Lake City on Thursday.
Meanwhile, Kirk's casket is to be flown from Utah to Arizona aboard Air Force Two on Thursday, an AP source says.
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Kirk, a close ally of US President Donald Trump, was killed with a gunshot from a distant rooftop at the Utah Valley University campus, where he was speaking Wednesday. Federal, state and local authorities were working what they called "multiple active crime scenes".
"This is a dark day for our state. It's a tragic day for our nation," Utah Governor Spencer Cox said on Wednesday.
"I want to be very clear this is a political assassination."
Two people were detained on Wednesday, but neither was determined to be connected to the shooting and both were released, public safety officials said.
The circumstances of the shooting drew renewed attention to an escalating threat of political violence in the United States that in the last several years has cut across the ideological spectrum.
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The assassination drew bipartisan condemnation, but a national reckoning over ways to prevent political grievances from manifesting as deadly violence seemed elusive.
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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 shot rings out, and Kirk can be seen reaching up with his right hand as blood gushes from the left side of his neck. Stunned spectators gasp and scream before people start running away.
Vice President JD Vance and his wife, Usha, were set to visit with Kirk's family on Thursday in Salt Lake City. According to a person familiar with Vance's plans but not authorised to speak about them publicly, the Vances will visit Utah instead of attending an outdoor ceremony to commemorate September 11 in New York.
Vance posted a remembrance on X chronicling their friendship, dating back to initial messages in 2017, through Vance's Senate run and ultimately praying after hearing of the shooting. Kirk played a pivotal role in setting up the second Trump administration, Vance wrote.
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"So much of the success we've had in this administration traces directly to Charlie's ability to organise and convene," Vance wrote.
"He didn't just help us win in 2024, he helped us staff the entire government."
Trump announced Thursday he'll posthumously award Kirk the Presidential Medal of Freedom.
Trump said he remained shocked by the horror of Kirk's assassination praised his ally's impact on conservative politics.
"Charlie was a giant of his generation, a champion of liberty and an inspiration to millions and millions of people," Trump said.
Kirk was taking questions about gun violence
Kirk was speaking at a debate hosted by his nonprofit political youth organisation, Arizona-based Turning Point USA, at the Sorensen Centre courtyard on campus. Immediately before the shooting, he was taking questions from an audience member about gun violence.
"Do you know how many transgender Americans have been mass shooters over the last 10 years?" the person asked. Kirk responded, "Too many."
The questioner followed up: "Do you know how many mass shooters there have been in America over the last 10 years?"
"Counting or not counting gang violence?" Kirk asked.
Then a shot rang out.
The shooter, who Cox pledged would be held accountable in a state with the death penalty, wore dark clothing and fired from a building roof some distance away.
Madison Lattin was watching only close by Kirk's left when she heard the bullet hit him.
"Blood is falling and dripping down, and you're just like so scared, not just for him but your own safety," she said.
She said she saw people drop to the ground in an eerie silence pierced immediately by cries. She and others ran. Some fell and were trampled in the stampede.
When Lattin later learned that Kirk had died, she wept, she said, describing him as a role model who had showed her how to be determined and fight for the truth.
Trump calls Kirk a 'martyr for truth'
About 3000 people were in attendance, according to a statement from the Utah Department of Public Safety. The university police department had six officers working the event, along with Kirk's own security detail, authorities said.
Trump announced Kirk's death on social media and praised the 31-year-old co-founder and CEO of Turning Point as "Great, and even Legendary". Later, he released a video in which he called Kirk a "martyr for truth and freedom".
Utah Valley University said the campus was evacuated after the shooting and will be closed until Monday.
Meanwhile, armed officers walked around the neighbourhood bordering the campus, knocking on doors and asking for any information residents might have on the shooting. Helicopters buzzed overhead.
Wednesday's event, billed as the first stop on Kirk's "The American Comeback Tour", had generated a polarising campus reaction. 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 was sparking controversy. He wrote, "What's going on in Utah?"
Condemnation from across the political spectrum
The shooting drew swift bipartisan condemnation as Democratic officials joined Trump, who ordered flags lowered to half-staff and issued a presidential proclamation, and other Republican allies of Kirk in decrying the violence.
"The murder of Charlie Kirk breaks my heart. My deepest sympathies are with his wife, two young children, and friends," said Gabrielle Giffords, the former Democratic congresswoman who was wounded in a 2011 shooting in her Arizona district.
In a joint statement, the Young Democrats of Connecticut and the Connecticut Young Republicans called the shooting "unacceptable".
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"We reject all forms of political violence," they said.
"There is no place in our country for such acts regardless of political disagreements."
The shooting appeared poised to become part of a spike of political violence that has touched a range of ideologies and representatives of both major political parties.
The attacks include the assassination of a Minnesota state lawmaker and her husband at their house in June, the firebombing of a Colorado parade in June 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 Pennsylvania campaign rally last year.
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