Convicted serial killer Lonnie Franklin Jr., otherwise known as the “Grim Sleeper,” died at the age of 67, People reported Sunday. Franklin was reportedly found unresponsive on Saturday night in his prison cell in San Quentin State Prison’s death row, and there were “no signs of trauma.”

“They don’t know why he died,” a California Department of Corrections and Rehabilitation spokeswoman said. An autopsy will reportedly be performed on Franklin by the Marin County coroner’s office.

In 2016, Franklin was charged with murdering 10 women over a period of 23 years that began in 1984. At the time, Franklin’s DNA was not logged in the national DNA database, but he was finally arrested in July 2010 after his 23-year-old son had to give his DNA to police when he was arrested for carrying a weapon.