A Pittsburgh police officer who was shot while off duty Sunday has died of his wounds, Chief Scott Schubert said in a statement Wednesday.

 “It is with great sorrow and a heavy heart that I must inform the public that Pittsburgh Bureau of Police Officer Calvin Hall passed away earlier today at UPMC Presbyterian,” the chief said.

Officer Hall, 36, was visiting friends early Sunday morning when he was shot three times in the back during an argument, the Post-Gazette reports.

“Regarding the investigation, we are investigating Officer Hall’s death as a homicide, and we are investigating the strong possibility, based on gathered evidence, that Officer Hall, who was off duty, was in fact acting under the color of the law when he was fatally shot,” Chief Schubert said.