The charges will be formally filed on Monday, officials said.

A private security guard who claimed self-defense in a fatal shooting following protests in Denver over the weekend will face second-degree murder charges, prosecutors announced Thursday.

Matthew Dolloff, 30, will be charged on Monday for the shooting death of Lee Keltner, said Denver District Attorney Beth McCann. Upon conviction, the felony charge carries a mandatory prison sentence of 16 to 28 years.

Dolloff was hired to protect a KUSA-TV news crew covering dueling protests in Denver’s Civic Center Park on Saturday. One, organized by a local right-wing group, was called a “Patriot Muster,” while a counterprotest by left-wing organizers was billed as a “BLM-Antifa Soup Drive.”

The shooting occurred in a courtyard near the Denver Art Museum following the demonstrations, authorities said. According to a police affidavit, Keltner, 49, a Navy veteran, was leaving the rally organized by the right-wing group Saturday afternoon when he ran into Dolloff. The shooting occurred after a “verbal altercation,” police said Saturday.

Photos of the incident taken by a photographer for the Denver Post showed Keltner initially appear to slap Dolloff in the face. Other photos taken by the Post photographer showed Dolloff drawing a semiautomatic handgun and aiming it at Keltner, who appeared to be spraying a can of pepper spray.

In another photo, Keltner is seen splayed on the ground, a can of pepper spray next to his body, as Dolloff continued to point his gun at him.

Police raced to the scene of the shooting and immediately took Dolloff into custody at gunpoint.

Police said two guns and a can of Mace were recovered at the scene. Officials declined to say earlier this week to whom the guns belonged.

Keltner’s death was ruled a homicide by the Denver Office of the Medical Examiner.

The Denver Police Department initially announced that the fatal shooting was being investigated as first-degree murder.

Dolloff claims he feared for his life and opened fire when the victim allegedly reached into his shirt, Dolloff’s lawyer told The Associated Press earlier this week.

KUSA-TV, the NBC affiliate in Denver, released a statement confirming Dolloff was hired through a private security company to protect its news crew covering the two demonstrations.

“It has been the practice of 9NEWS for a number of months to hire private security to accompany staff at protests,” KUSA said in the statement.

Denver investigators refuted rumors that Dolloff was a member of Antifa, a militant left-wing political movement.

ABC News’ Bill Hutchinson and Clayton Sandell contributed to this report.