The FBI has released photos of someone who they have described as a "person of interest" in the shooting of influential conservative activist Charlie Kirk.
FBI officials in the Salt Lake City office did not say the person was the suspected shooter.
They released two pictures of someone in a baseball cap, dark glasses and casual clothing, It appealed "for the public's help identifying this person of interest in connection with the fatal shooting".
Police and US federal agents earlier said they found the bolt-action rifle they believed was used to kill Mr Kirk during a university appearance in Utah, but were still hunting the shooter.
Mr Kirk, a 31-year-old podcast-radio commentator and a close ally of US President Donald Trump, is credited with helping build the Republican Party's support among younger voters.
Watch: Eyewitness video of moments surrounding shooting of Charlie Kirk
He was killed by a single gunshot in what Utah Governor Spencer Cox called a political assassination.
The killing, captured in graphic detail in videos that rapidly spread around the internet, occurred as Mr Kirk spoke onstage at an outdoor event called "Prove Me Wrong" in front of about 3,000 people at Utah Valley University in Orem, Utah, about 65km south of Salt Lake City.
The killer arrived on campus a few minutes before the event began, and could be seen on security-camera video ascending stairwells to get onto a nearby roof before firing a single shot, according to the FBI and state officials.
The shooter jumped off the roof and fled into an adjoining neighborhood, Robert Bohls, the FBI special agent in charge, told reporters. Investigators found a "high-powered, bolt-action" rifle in a nearby wooded area, and were examining that along with palm prints and footprints for clues.
Watch: FBI say rifle recovered in Kirk shooting, suspect is of 'college age'
The shooter appears to be of college age and "blended in well" on the campus, Utah Public Safety Commissioner Beau Mason told reporters.
Mr Kirk, co-founder and president of the conservative student group Turning Point USA, was pronounced dead at a local hospital hours later. His killing stirred immediate expressions of outrage and denunciations of political violence from Democrats, Republicans and foreign governments.
Mr Cox said Mr Kirk's events on college campuses were part of a tradition of open political debate that was "foundational to the formation of our country, to our most basic constitutional rights".
"When someone takes the life of a person because of their ideas or their ideals, then that very constitutional foundation is threatened," Mr Cox said.
Vice President JD Vance canceled his trip to New York to commemorate the 9/11 attacks and instead will travel to Utah to visit Mr Kirk's family, a person familiar with the situation said.
Mr Kirk began his career in conservative politics as a teenager. A little more than a decade later, some of the friends he made along the way are now at the highest levels of US government and media, with Mr Vance recalling that he was in multiple group chats with Mr Kirk.
Watch: Trump says killing of Charlie Kirk a 'dark day for America'
"So much of the success we've had in this administration traces directly to Charlie's ability to organize and convene," Mr Vance wrote in a lengthy tribute posted on social media. "He didn't just help us win in 2024, he helped us staff the entire government."
The shooting punctuated the most sustained period of US political violence since the 1970s.
Reuters has documented more than 300 cases of politically motivated violent acts across the ideological spectrum since supporters of Mr Trump attacked the US Capitol on 6 January 2021.
Mr Trump himself has survived two attempts on his life, one that left him with a grazed ear during a campaign event in July 2024 and another two months later foiled by federal agents.
Two people were detained, questioned and released yesterday evening, but neither were suspects, the FBI said.
One of the two detainees, an older man seen in photos that circulated online shortly after the killing, was familiar to locals as a political "gadfly," according to the Salt Lake Tribune.
Officials said he had been charged with obstruction by university police and released.
Mr Kirk, who was married and the father of two young children, had just returned to the US from an overseas speaking tour in South Korea and Japan.
His appearance was part of a planned 15-event "American Comeback Tour" of US college campuses.
Known for his often-provocative discourse on race, gender, immigration and gun regulation, Mr Kirk often used such events to invite members of the crowd to debate him live.
Read more: Conservative influencer Charlie Kirk played key role in Trump's re-election
"He would go into these hostile crowds and answer their questions," Mr Vance wrote in his tribute. "If it was a friendly crowd, and a progressive asked a question to jeers from the audience, he'd encourage his fans to calm down and let everyone speak."
At the moment he was shot, Mr Kirk, a staunch advocate of the US Constitution's Second Amendment right to bear arms, was being questioned by an audience member about gun violence, according to multiple videos of the event posted online.
In a video message taped in the Oval Office, Mr Trump vowed that his administration would track down those responsible for Mr Kirk's killing.
Mr Trump, who routinely describes political rivals, judges and others who stand in his way as "radical left lunatics" and warns that they pose an existential threat to the nation, also decried violent political rhetoric, while casting it as a phenomenon of the political left.
"For years, those on the radical left have compared wonderful Americans like Charlie to Nazis and the world's worst mass murderers and criminals," Mr Trump said in the video.
"This kind of rhetoric is directly responsible for the terrorism that we're seeing in our country today, and it must stop right now."
