SALT LAKE CITY — A demonstrator was shot and killed at Salt Lake City’s “No Kings” protest when a man believed to be part of the event’s peacekeeping team fired at another man allegedly aiming a rifle at protesters, authorities said Sunday.

Police took the alleged rifleman, Arturo Gamboa, 24, into custody Saturday evening on a murder charge, Salt Lake City Police Chief Brian Redd said at a Sunday news conference. The bystander, Arthur Folasa Ah Loo, 39, died at the hospital.

Detectives don’t yet know why Gamboa pulled out a rifle or ran from the peacekeepers, but they accused him of creating the dangerous situation that led to Ah Loo’s death. The Associated Press did not immediately find an attorney listed for Gamboa or contact information for his family in public records.

Redd said a man dressed in a brightly colored vest fired three shots from a handgun at Gamboa, inflicting a relatively minor injury but fatally shooting Ah Loo.

The gunshots sent hundreds of protestors running, some hiding behind barriers and fleeing into parking garages and nearby businesses, police said in a statement. “That’s a gun. Come on, come on, get out,” someone can be heard saying in a video posted to social media that appears to show the events.

“No Kings” protests swept across the country Saturday, and organizers said millions rallied against what they described as President Donald Trump’s authoritarian excesses. Confrontations were largely isolated.

The Utah chapter of the 50501 Movement, which helped organize the protests, said in a statement on Instagram that they condemned the violence.

The Utah chapter did not immediately respond to AP questions about the peacekeeping team. It was unclear who hired them, whether they were volunteers or what their training was prior to the event. Redd said that the peacekeepers’ actions are also part of the investigation.

The shooter and another person in a vest allegedly saw Gamboa separate from the crowd of marchers in downtown Salt Lake City, move behind a wall and withdraw a rifle around 8 p.m., Redd said.

When the two men in vests confronted Gamboa with their handguns drawn, witnesses said Gamboa raised his rifle into a firing position and ran toward the crowd, said Redd.

That’s when one of the men dressed in the bright vests shot three rounds, hitting Gamboa and Ah Loo, said Redd. Gamboa, who police said didn’t have a criminal history, was wounded and treated before being booked into jail.

Police said they recovered an AR-15 style rifle, a gas mask and a backpack at the scene.