One of California’s most notorious killers is back in the spotlight as a new documentary revisits his crimes and uncovers even darker secrets.

Joseph Naso, the former photographer convicted in 2013 of killing four women, is now claiming he actually killed 26 women. The bombshell confession comes from a fellow death row inmate, William Noguera, who spent more than a decade building trust with Naso inside California’s infamous San Quentin State Prison.

The chilling revelations are featured in a new Oxygen documentary, “Death Row Confidential: Secrets of a Serial Killer,” premiering Sept. 13.

“He’s guilty of more than anyone knows,” Noguera says during a preview of the new series. “He told me everything, and I wrote all of it down.” 

Investigators also uncovered a diary detailing more than 100 sexual assaults dating back to the 1950s, many involving underage girls. With the new claims emerging, law enforcement agencies across California and the FBI are now re-examining unsolved cases.

“Our two minds, cop and convict, working together. I know that I can solve unsolved murders. Let’s get them,” Mains said. 

As the documentary reveals, Naso’s crimes may stretch further than anyone ever imagined — and his disturbing legacy continues to haunt both investigators and families seeking closure.

Stepheny Price covers crime including missing persons, homicides and migrant crime. Story tips: stepheny.price@fox.com.