What is the fine-grained topic of the following text: He spent his last decade on the force in the cold case unit, digging through old unsolved murder files for new clues and testable biological evidence, and while Shepard and his colleagues helped capture some of southern California’s most infamous bad guys—Rodney Acala, Chester Turner, and Lonnie David Franklin Jr. (aka the Grim Sleeper), to name a few—the cases that were never solved continue to nag Shepard in his retirement. Only return the topic.
A retired detective's experiences working on cold cases in southern California