Denver District Attorney Beth McCann said in a written statement that she will file the murder charge against Matthew Dolloff, 30, in the shooting death of Lee Keltner on Saturday.

Dolloff has been held without bond in the Denver County jail since his arrest, jail records show.

A lawyer for the Dolloff family said he acted in self defence, local media reported this week.

The shooting erupted in a courtyard near an art museum in downtown Denver following a “Patriot Rally” and a counter-demonstration dubbed a “BLM-Antifa Soup Drive”.

Dolloff opened fire with a semi-automatic handgun after Keltner hit him in the face and sprayed him with a chemical agent following a “verbal altercation” between the two men, as demonstrators from both sides were dispersing, police said.

An image of the incident captured by a Denver Post photographer showed Keltner spraying the chemical at Dolloff, who was pointing a gun at Keltner.

Keltner, 49, died from a single gunshot wound, authorities said.

Keltner’s son told the Denver Post his father was not affiliated with any group, but was at the rally to support police.

Dolloff was hired through the Pinkerton agency to provide security for a crew from KUSA-TV, an NBC News affiliate, the station said in a statement, adding it requested that security personnel be unarmed.

Pinkerton said on Twitter that Dolloff was not an employee “but rather a contractor agent from a long standing industry vendor”.

If convicted of second-degree murder, Dolloff faces a mandatory sentence of between 16 and 48 years in prison, McCann said.