DENVER — A television station security guard accused of fatally shooting a pro-police demonstrator following opposing rallies will be charged with second-degree murder, Denver's District Attorney Beth McCann said Thursday.

The charge will be filed against Matthew Dolloff, 30, on Monday — the deadline for prosecutors to bring charges — in the death of Lee Keltner, 49, McCann said in a statement. She did not elaborate on her decision to prosecute Dolloff, who was protecting a KUSA-TV producer at the time, for second-degree murder, rather than the first-degree murder charge he was arrested on suspicion of.

People convicted of second-degree murder face a mandatory sentence of between 16 and 48 years in prison.

No attorney was listed for Dolloff yet in court records.

Police say Keltner was in a verbal dispute with a 27-year-old man as the rallies broke up Saturday when Dolloff and a 25-year-old person got into an altercation with Keltner.

Keltner slapped Dolloff in the head and Dolloff pulled out a semiautomatic handgun and shot Keltner as Keltner discharged pepper spray at him, police said in an arrest affidavit.

A cell phone video taken by KUSA's producer suggests that Keltner was upset that his dispute with the first man was being recorded by cameras.

It shows Keltner in a confrontation with a man wearing a T-shirt that read, “Black Guns Matter.” A bystander is trying to defuse the argument, which occurred after a “Patriot Muster” demonstration and “BLM-Antifa Soup Drive” counterprotest downtown.

The video then shows Keltner, holding a spray can, walking out of view. A man’s voice — it’s unclear if it’s Keltner — is heard saying the area was no place for cameras.

“Get the cameras out of here or I’m going to f—- you up,” the unidentified man says. Keltner and Dolloff are then shown scuffling before the video stops.

The producer resumes filming after the shooting and tells arriving police that he is with the press and says of the man who was shot, “That guy was going to get me.” He also says the security guard shot Keltner because Keltner used mace.

Someone out of view can also be heard saying “he's got magazines in his coat” but it is not clear who the person is talking about.

Police said they found two guns but they have not explained who they belonged to.