DENVER — A private security guard hired by a Denver television news station was being held by the authorities in connection with a fatal shooting that happened on Saturday after opposing rallies between far-right and far-left activists.
The guard, Matthew Dolloff, 30, was being investigated for first-degree murder, the Denver Police Department said on Twitter on Sunday. He was being held in the Denver County Jail, according to court records.
The shooting occurred at 3:37 p.m. local time, south of Civic Center Park, near the Denver Art Museum, as a right-wing “patriot muster” and a counterprotest “soup rally” organized by a left-wing group, the Denver Communists, began winding down.
The police kept the groups on separate sides of the park, which was divided by an amphitheater and temporary fencing. Some members of the right-wing group filmed and heckled the left-wing activists from across the fencing, though the groups did not engage in a large-scale verbal or physical confrontation, said Chelsea Jacobs, a member of the Denver Communists.
It was unclear whether Mr. Dolloff, whose surname is spelled in jail records as Doloff, had been charged on Sunday. He had a court hearing on Sunday morning, records show, but an arrest affidavit was sealed. The district attorney did not respond to requests for comment and it was unclear if Mr. Dolloff had a lawyer.
Randy Corporon, a lawyer and Republican national committeeman, identified the victim as Lee Keltner. He said he had met Mr. Keltner at the protest on Saturday before the shooting.
Mr. Keltner owned a custom hat business, Crossfire Hats, in Brighton, Colo., about 20 miles northeast of Denver, The Denver Post reported in 2015.
“I love creating new designs and making people happy when they get the hat that they have been searching for,” he said.
Mr. Keltner, who was identified by the police only as a white man, was taken to the hospital, where he died, Joe Montoya, chief of the Police Department’s investigations division, said at a news conference on Saturday.
The medical examiner is expected to identify the body on Monday, a spokeswoman, Tammy Vigil, said.
One of the organizers of the right-wing protest, John Tiegen, said on Instagram on Sunday that he was “deeply saddened to learn of the tragic death of a fellow American patriot.”
A Denver news channel, 9News, reported that a station producer and a contractor had been taken into custody.
“A private security guard who was hired by 9News is the suspect detained by DPD,” the station reported. “It has been the practice of 9News for a number of months to hire private security to protect staff at protests.”
The security guard was contracted through the company Pinkerton, Mark A. Cornetta, president and general manager of 9News, said on Saturday. He said the producer had been released.
Mr. Dolloff has never held the required license with Denver to work as a security guard, said Eric Escudero, a spokesman for the city and county licensing department. “If he was operating as a security guard, he was in violation of the law,” he said.
While Pinkerton had an active license in the city and county of Denver, the company “is legally responsible” for ensuring that its employees are licensed, Mr. Escudero said.
The city has opened an administrative investigation into the matter, he said.
In a statement, Pinkerton said on Sunday that it was gathering information and fully cooperating with law enforcement. “We take loss of life in any situation very seriously,” the statement said.
Later Sunday, the company added: “The agent is not a Pinkerton employee but rather a contractor agent from a third-party vendor. The vendor is responsible for proper licensing for all agents under their employment. It is the responsibility of the vendor to ensure any agents assigned to security projects have the required documentation execute the assignment.”
In the moments before the shooting, the victim, who was wearing sunglasses and a face covering and holding a canister of Mace or a similar substance, had gotten into a loud, heated argument with a man wearing a “Black Guns Matter” shirt, videos posted on social media and photographs by The Denver Post show.
The victim is seen in the photos striking Mr. Dolloff and spraying at him while Mr. Dolloff points a firearm. The victim is then seen splayed on the ground as Mr. Dolloff holds the firearm.
Another video shows several police officers rushing to the scene, shouting, “Drop the gun!” Mr. Dolloff can be seen kneeling on the sidewalk as officers surround him.
Hiring a private guard or security adviser for journalists is a relatively common practice overseas and domestically, Robert Mahoney, the deputy executive director of the Committee to Protect Journalists, said on Sunday.
Internationally, these guards or advisers have been used during wars since the 1990s and most are unarmed, he said. In Iraq, for instance, some journalists became the target of kidnappings.
These security experts often serve as a guide to journalists, he said, and help get them out of potentially dangerous situations if what they are covering takes a turn for the worse.
Bryan Pietsch reported from Denver, and Christina Morales from Miami. Concepción de León contributed reporting from New York.