The unlicensed security guard being held in connection with a weekend slaying after a pair of rallies in Civic Center park will be charged with second-degree murder, authorities announced Thursday.

Matthew Dolloff, 30, who was arrested for fatally shooting 49-year-old Lee Keltner on Saturday, will be formally charged Monday, according to the office of Denver District Attorney Beth McCann.

According to a news release, the charge carries a mandatory prison sentence of 16 to 48 years. The district attorney’s office did not release further details of the pending charges.

Dolloff was acting as a private security guard for Denver Gazette news partner 9News at the rallies, the company confirmed the day of the shooting, and was contracted through Pinkerton. Management for the station released a statement Tuesday saying no one at the company was aware Dolloff was armed. The Denver Gazette found Dolloff does not hold a license to work as a private security guard in Denver.

An unidentified 9News producer was also detained immediately following the shooting, but was released Saturday and is not a suspect in the incident.

Pinkerton said in an earlier statement it does not employ Dolloff, who is a “contractor from a long standing industry vendor.” Pinkerton did not name the vendor.

Colorado has several levels of homicide charges types, ranging from first-degree murder as the most serious, followed by second-degree murder, manslaughter and criminally negligent homicide.

Stan Garnett, who served as Boulder’s district attorney until 2018 and is now a shareholder at Brownstein Hyatt Farber Schreck, explained a charge of second-degree murder is often used in situations when a person made a split-second decision and didn’t have time to deliberate over their action.

Garnett said the differentiator between murder in the first and second degrees is nuanced, but crucial: First-degree murder requires premeditation and intent to kill, while a second-degree charge involves murder that is knowing. In other words, a defendant didn’t intend to kill the victim, but probably knew the action they took would kill someone.

He said the choice to charge Dolloff with second-degree homicide doesn’t surprise him.

“It would be very tough [to prove] in this very quick environment that he had deliberation and he intended to kill,” Garnett said.

He added a self-defense argument is an affirmative defense, so attorneys for Dolloff will have to raise it themselves during the case and the district attorney does not need to take it into account when filing charges.

On Tuesday, Denver Police released a probable cause statement for Dolloff’s arrest.

The affidavit says Keltner and another man, whose name is redacted, had a verbal confrontation as Keltner and others left a “Patriot Muster” rally through the courtyard between the Denver Art Museum and the Denver Public Library.

A third man with his name redacted by the affidavit put himself between the two men in an attempt to break up the dispute.

Dolloff and an unidentified person then got involved in a confrontation with Keltner and another person, whose name is redacted, and Keltner turned his attention toward Dolloff.

According to the affidavit, video footage and still images show Keltner striking Dolloff in the side of the head. Dolloff then drew his handgun from his waistband and shot Keltner once as Keltner pepper-sprayed Dolloff, according to the affidavit.

Keltner owned Crossfire Hats in Brighton, according to a 2015 Denver Post business profile.

Saturday afternoon’s rallies were held by opposing groups.

The Patriot Muster rally was organized by John “Tig” Tiegen, a man from El Paso County who has proclaimed support for the Second Amendment and President Donald Trump.

The other event billed itself as a “BLM-Antifa Soup Drive,” and organizers said they showed up in opposition to the Patriot Muster rally.

The Elbert County sheriff has confirmed that Dolloff’s concealed carry permit was revoked Monday pending the criminal investigation.