S4, E16: Alexx comes under suspicion after a convicted child molester is found murdered in her neighborhood. Horatio finds DNA evidence that shows that Alexx was putting up posters in her neighborhood warning parents that a child molester was moving to the area. The CSIs soon discover that a gang-mentality had spread throughout the community and that any number of the residents would have liked the man gone. Now, the team must work to prove that Alexx didn't cause the man's murder.