ST. LOUIS (AP) — A man who was 16 years old when he fatally shot a retired St. Louis police sergeant has been sentenced to 20 years on prison.

The retired officer, Ralph E. Harper, was 67 when he was killed during a carjacking attempt in October 2018. Jalynn Garner, 21, pleaded guilty to second-degree murder, robbery and two counts of armed criminal action and was sentenced Monday.

During the robbery attempt, Harper fired his police revolver, striking Garner in the arm, prosecutors said. Harper died in the exchange of gunfire. Garner was arrested and certified to stand trial as an adult.

An accomplice who was 15 at the time of the crime, Justin Matthews, pleaded guilty to second-degree murder, robbery and other charges in 2019. He said he provided Garner with a gun and waited in a stolen SUV as Garner tried to rob Harper.

Matthews received a suspended 20-year sentence and was placed in a juvenile detention center until age 21, when he will be eligible for parole if he completes a juvenile detention program.

Harper retired from the police department in 2007 after a 33-year career.