John "Sustained" Chastain (d. April 1999) was a detective with the Los Angeles Police Department's Internal Affairs Division. Chastain and Bosch are physically similar but polar opposites in their approach to police work. They had a long running mutual dislike that continued until Chastain was killed. In the early 1990s, Chastain began passing information on internal investigations, including cases that were ruled unfounded, to civil rights attorney Howard Elias under the codename "Parker."