CHICAGO (STMW) — A 17-year-old has been charged as an adult with murder, in connection with the robbery of an off-duty Cook County Sheriff’s sergeant Monday night that left another suspect dead.

Devante Graham of the 10300 block of South Corliss faces one count of first-degree and one count of armed robbery, according to Cook County State’s Attorney’s office spokeswoman Tandra Simonton.

About 9:55 p.m. Monday, three males tried to rob the sergeant as he was pumping gas at a Citgo station in the 700 block of East 103rd Street, according to Chicago Police and Cook County Sheriff’s spokesman Ben Breit.

They demanded the sergeant’s wallet, which he gave them. One of the suspects then pulled a handgun and aimed it at the sergeant, according to police. The sergeant then pulled his own gun and shot the suspect in the head, killing him.

Deonta Mackey, 16, of the 900 block of East 104th Street, was pronounced dead at the scene at 10:08 p.m., according to the Cook County Medical Examiner’s office. An autopsy Tuesday found he died of multiple gunshot wounds and his death was ruled a homicide.

Police said the suspect’s weapon was recovered at the scene.

The third suspect is still being sought, police said.

Graham will appear in bond court Thursday, Simonton said.

(Source: Sun-Times Media Wire © Chicago Sun-Times 2014. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)