Popular American right-wing influencer Charlie Kirk has died after being shot while speaking at a university campus in Utah.
US President Donald Trump confirmed the 31-year-old's death in a post on social media.
Later, in a video message, he said he was "filled with grief and anger at the heinous assassination".
"Charlie inspired millions and, tonight, all who knew him and loved him are united in shock and horror."
Mr Kirk was shot while addressing a crowd at Utah Valley University in Orem, south of Salt Lake City, just after midday, local time.
"I want to be very clear that this is a political assassination," Utah Governor Spencer Cox, a Republican, told a news conference.
"And I just want to remind people that we still have the death penalty here in the state of Utah.
"Our nation is broken."
On Wednesday evening, local time, Mr Cox said a "person of interest" had been taken into custody and was being interviewed.
But less than 2 hours later, FBI director Kash Patel said that person had been released "after an interrogation by law enforcement".
Another suspect arrested earlier was also found not to be the shooter, but was booked for obstruction of justice.
"Our investigation continues," Mr Patel said.
As of 10pm Wednesday US time, authorities were searching for a new person of interest, a law enforcement official familiar with the matter told the Associated Press.
Video from the event shows Mr Kirk speaking into a microphone while seated under a white tent when he is shot in the neck.
Police said about 3,000 people had gathered to watch him speak. He was answering a question about mass shootings when he was shot.
A university spokesperson told The New York Times the fatal shot had been fired from a campus building almost 200 metres away.
Mr Kirk, a father of two, was the founder of conservative youth organisation Turning Point USA.
He had millions of followers on social media and a top-rating political podcast. He was widely credited with helping to mobilise a significant youth vote for Mr Trump at last year's election.
In his video message, the president said Mr Kirk's death marked "a dark moment for America".
"It's long past time for all Americans and the media to confront the fact that violence and murder are the tragic consequence of demonising those with whom you disagree," Mr Trump said.
He said his administration would "find each and every one of those who contributed to this atrocity, and to other political violence, including the organisations that fund it".
Donald Trump Jr, the president's son, said Mr Kirk was "like a little brother to me".
"The impact he had on young people — reaching them in masses, giving them courage to stand up, to think for themselves, and to fight for freedom — is immeasurable.
"There is no question that Charlie's work and his voice helped my father win the presidency."
Mr Kirk was visiting Utah as part of a two-month tour of university campuses, which he had called The American Comeback Tour, to hold public debates with progressive students.
His online content, including clips from the so-called "Prove Me Wrong" campus debates, frequently went viral.
America's living past presidents all used their online platforms to appeal against political violence.
"Violence and vitriol must be purged from the public square," George W Bush said. Joe Biden and Barack Obama both said such violence had "no place" in America.
Bill Clinton said: "I hope we all go through some serious introspection and redouble our efforts to engage in debate passionately, yet peacefully."
US Health Secretary Robert F Kennedy Jr, whose senator father and president uncle were famously assassinated in the 1960s, said: "Once again, a bullet has silenced the most eloquent truth teller of an era.
"My dear friend Charlie Kirk was our country's relentless and courageous crusader for free speech," Mr Kennedy said.
California's Democratic Governor Gavin Newsom, who recorded a podcast with Mr Kirk earlier this year, called the shooting "disgusting, vile and reprehensible".
"In the United States of America, we must reject political violence in every form," he said.
Mr Trump has ordered flags to be flown at half-mast at public sites across the US for the rest of the week.
