Virginia police say an off-duty U.S. Park Police officer fatally shot another off-duty officer in an apartment in what local authorities are preliminarily describing as an unintentional firing of a gun.

Fairfax County detectives determined that the officer pulled the trigger on a gun he believed was unloaded early Sunday, striking Jesse Brown Hernandez inside an apartment in McLean, Virginia, according to a police statement.

Hernandez, 22, was found deceased with a gunshot wound to the upper body, Fairfax County police said.

The officer who fired the gun has been charged with involuntary manslaughter, police said. He is being held at the Fairfax County Adult Detention Center on no bond.

Police said that preliminarily detectives determined that Roy “was attempting to dry fire a firearm.”

“Dry firing is when one simulates the discharge of a firearm by pulling the trigger of a firearm that is unloaded,” police said in the news release.

There were four people in the apartment at the time of the shooting, and all were known to each other, police said. Three of the four involved individuals were off-duty U.S. Park Police officers.