A man who was believed to be part of a peacekeeping team for the "No Kings" protest in Salt Lake City shot at a person who was brandishing a rifle at demonstrators, striking both the rifleman and a bystander who later died at the hospital, 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 was Arthur Folasa Ah Loo, 39, a fashion designer from Samoa.

"Our detectives believe he was an innocent bystander," the police department said in a social media post.

Detectives don't yet know why Gamboa pulled out what they describe as an AR-15-style rifle, or why he 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 a lawyer listed for Gamboa or contact information for his family in public records.

Redd said the man believed to be part of the peacekeeping team, dressed in a neon green vest, fired three shots from a handgun at Gamboa, inflicting a relatively minor injury but fatally shooting Ah Loo. Redd did not share the man's name.

Police still probing who fired shots

Volunteer peacekeeping teams are common for protests, said Sarah Parker, a national co-ordinator for 50501 Movement, which was a partner in the "No Kings" protest. But the organizers ask attendees, including the peacekeepers, not to bring weapons, she said. Still, Parker said they stopped what could've been a larger mass casualty event.

"Our safety team did as best as they could in a situation that is extremely sad and extremely scary," said Parker.

Ah Loo, known as Afa, was a husband and father to two children and a fashion designer who leaned into his Samoan heritage, according to a GoFundMe page organized to support his family. Ah Loo founded Creative Pacific, an event celebrating the diversity of the Pacific Islands, with workshops, artists and a fashion runway. He was on Season 17 of the fashion design reality TV show Project Runway.

The gunshots sent hundreds of protesters 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 U.S. on Saturday, and organizers said millions rallied against what they described as President Donald Trump's authoritarian tendencies.

The shooter and another person in a neon 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 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.

Redd said that the peacekeepers' actions are also part of the investigation.

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