A teenager charged in the shooting death of a retired St. Louis police sergeant will be tried as an adult. Sixteen-year-old Justin Mathews was 15 when he was charged with second-degree murder and other crimes in the October death of retired Sergeant Ralph Harper. Prosecutors say Harper was parking his car when he was approached by another teenager, who was armed. Harper was killed after exchanging gunfire with the robber. The St. Louis Post-Dispatch reports Judge Steven Ohmer decided to charge Mathews as an adult, following the recommendation of a juvenile officer. Mathews is accused of driving the shooter to the scene and providing him with a gun, then fleeing in a stolen car. Harper worked for the police department for 33 years before he retired in 2007.