An off-duty Chicago Police officer fatally shot a 17-year-old boy who was robbing him at gunpoint Tuesday night in south suburban Hazel Crest, police said.

Hazel Crest officers responded at 9:54 p.m. to a call of a robbery and shots fired on Birchwood Drive, according to Hazel Crest police. When they arrived, they found one person standing with his hands in the air, identifying himself as a Chicago Police officer; and a 17-year-old boy lying face-down and unresponsive between two houses. Money was scattered around the area.

A preliminary investigation revealed the officer had met with two people, planning to buy a computer in an arrangement set up online, police said. He was off-duty at the time, driving his personal vehicle and wearing street clothes.

When the officer arrived in Hazel Crest, he exited his vehicle and two males walked up, saying they had the computer, police said. One of the suspects then pulled out a handgun.

While holding the officer at gunpoint, the pair began stealing items from his pockets, and eventually found his badge and gun on his belt under his shirt, police said. They took his badge, but before they could get the gun, the officer fired shots at both suspects.

One of the suspects, identified as 17-year-old Aaron Brandon, fell between two houses nearby, according to police and the Cook County medical examiner’s office. He had a gun in his hand, and the officer’s CPD badge was found on the ground nearby. A key fob for the officer’s vehicle was found in his pocket, police said.

Brandon, a Homewood resident, was pronounced dead at South Suburban Hospital in Hazel Crest at 10:42 p.m., according to the medical examiner’s office. An autopsy Thursday found he died of multiple gunshot wounds, and his death was ruled a homicide.

The other suspect ran away, police said. The officer’s weapon was on the hood of his car.

Hazel Crest detectives are investigating the armed robbery, while the Illinois State Police Public Integrity Unit is investigating the officer’s use of force.