SAN QUENTIN, Calif. (AP) — A former California police officer turned serial killer who was on death row after being convicted of murdering six people in the 1980s has died of natural causes, authorities said.

Anthony Sully, 79, died Friday at a medical facility outside the San Quentin Rehabilitation Center, where he had been housed for decades, according to the California Department of Corrections and Rehabilitation.

The Marin County Coroner’s Office will determine Sully’s official cause of death, the department said in a news release Monday.

Sully was sentenced to death in June 1986 for the slayings of Kathryn Barrett, 24; Barbara Searcy, 22; Gloria Jean Fravel, 24; Brendan Oakden, 19; Michael Thomas, 24; and Phyllis Melendez, 20.

The victims were beaten, stabbed and shot inside an electrical supply warehouse in the San Francisco Bay Area in 1983. Three of the bodies were found stuffed into barrels dumped at Golden Gate Park in San Francisco, the San Jose Mercury News reported. Detectives found his fingerprints on some of the bodies.

Sully was a Bay Area police officer from 1966 to 1974.

He maintained at his sentencing that he did not get a fair trial, telling the judge, “I am not a monster, not a maniac, not subhuman,” according to the Mercury News, citing news accounts at the time.