CSI - Spoilers

Episode 8.12 - Grissom's Divine Comedy: A man named Don Cook is set to testify in the murder trial a gangbanger after he witnessed the man fleeing the scene of the crime. When Cook is murdered, Deputy District Attorney Madeline Klein is positive that the gangbanger killed him. She puts Grissom on the case but his investigation reveals that someone close to Cook was responsible for his murder. He also helps bust the gangbanger for passing along instructions for other crimes via prison library books. Source: SpoilerFix.com