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 December of 1992, he investigated Narcotics detective Calexico Moore, and was assigned to investigate Moore's death when the Narcotics detective was found dead at the Hideaway motel in an apparent suicide.
In September of 1996, he sat in on a meeting between Harry Bosch and four federal agents when an undercover agent accused Bosch of planting evidence to frame him for the murder of Anthony Aliso. Chastain later interviewed Bosch himself regarding his investigation in Las Vegas and his relationship with former FBI agent and convicted felon Eleanor Wish.
In 1998 Chastain investigated several detectives from RHD who were accused of brutality against Michael Harris, a murder suspect who was later acquitted. Chastain was going to file charges against at least one of them, but someone higher up in the department told him to drop it. In April of 1999, Chastain was subpoenaed by Elias to testify on behalf of the plaintiff in Harris' civil suit against the LAPD. Chastain attempted to refuse the subpoena, but was served anyway, and realized that being outed as Elias' source within the Department would ruin his career. He then followed Elias from the lawyer's office on a Friday night and shot the attorney as he boarded the Angels Flight train, after which he shot and killed a witness already on the train. He was later called back to the scene of the murder as an investigator along with a team of internal affairs detectives, and was partnered with Bosch on the investigation.
To deflect suspicion from himself, Chastain took bullets from the evidence locker relating to a shooting by Frank Sheehan five years previously. He submitted these bullets for ballistic testing to implicate Sheehan in the murder of Elias. Chastain then murdered Sheehan at Bosch's house, making it look like suicide.
The murder of Elias had triggered several days of rioting and unrest. That Monday night, Bosch arrested Chastain and was attempting to take him to Parker center when their car was surrounded by rioters. Chastain was dragged from Bosch's car and killed. The department chose to portray Chastain as a hero and Sheehan as the killer of Elias.