Charlie Kirk, a conservative American activist and influential ally of United States President Donald Trump, has been shot dead at a university event in Utah in an apparent targeted assassination.

Trump announced the death of Kirk, the 31-year-old cofounder of activist youth group Turning Point USA, in a social media post on Wednesday.

“The Great, and even Legendary, Charlie Kirk, is dead,” Trump wrote on his Truth Social platform.

“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.”

Trump said he had ordered flags across the US to be lowered to half-mast until Sunday in Kirk’s honour.

In a video address from the White House later on Wednesday, Trump blamed rhetoric from the “radical left” as a factor in Kirk’s murder.

“It is 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, day after day, year after year, in the most hateful and despicable way possible,” Trump said.

Utah authorities said Kirk was killed with a single shot in what they believe was a targeted attack.

Videos circulating online show Kirk addressing a large outdoor crowd at Utah Valley University, a public institution located about 63km (40 miles) from Salt Lake City, when a shot rings out.

Kirk is seen recoiling and raising his hand to his neck as he falls off his chair, sending the attendees running.

Jeb Jacobi, a volunteer for Turning Point USA who witnessed the shooting, described the attack as shocking.

“I was on the third row from the bottom. I saw Charlie collapse in a pool of blood, and he was dragged off and carried over to the car. It was shocking to see,” Jacobi told Al Jazeera.

“It was one of the most shocking things I’ve ever seen.”

“People were panicking, people were screaming, people were running in all different directions,” Jacobi added.

“It was awful. I started panicking. I called my family, I called my parents, I called my aunt, my sister. I talked to everyone.”

Utah Governor Spencer Cox called the shooting a “political assassination”.

“This is a dark day for our state. It is a tragic day for our nation,” Cox said at a news conference.

Cox condemned the apparent assassination as a threat to Americans’ right to discuss and debate ideas openly.

“Charlie Kirk was first and foremost a husband and dad to two young children,” Cox said.

“He was also very much politically involved, and that’s why he was here on campus. Charlie believed in the power of free speech and debate to shape ideas and to persuade people. Historically, our university campuses in this nation, and here in the state of Utah, have been the place where truth and ideas are formulated and debated.”

Democrats, including former Presidents Joe Biden and Barack Obama, joined the condemnation.

“There is no place in our country for this kind of violence,” Biden said.

“It must end now. Jill and I are praying for Charlie Kirk’s family and loved ones.”

California Governor Gavin Newsom called Kirk’s assassination “disgusting, vile, and reprehensible”.

“In the United States of America, we must reject political violence in EVERY form,” Newsom said on X.

Beau Mason, commissioner of the Utah Department of Public Safety, said authorities were searching for a suspect “dressed in all dark clothing” who may have opened fire from a roof.

“The only information we have on the suspect, the possible shooter, is taken from closed-circuit TV here on campus. We do have that. We’re analysing it. But it is security camera footage, so you can kind of guess what the quality of that is,” Mason said at a news conference.

FBI director Kash Patel said a person who had been detained earlier was released following an interrogation by law enforcement.

“Our investigation continues and we will continue to release information in interest of transparency,” Patel said on X.