"They don't know why he died," California Department of Corrections and Rehabilitation spokeswoman Terry Thornton tells PEOPLE

Convicted “Grim Sleeper” serial killer Lonnie Franklin Jr. has died. He was 67.

Frankin, who preyed on South Los Angeles women for more than two decades, was found unresponsive at 7:20 p.m. on Saturday evening in his prison cell on San Quentin State Prison’s death row.

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“There were no signs of trauma,” California Department of Corrections and Rehabilitation spokeswoman Terry Thornton tells PEOPLE. “They don’t know why he died.”

Prison medical staff rendered aid and summoned an ambulance. He was pronounced dead at 7:43 p.m.

Thornton says an autopsy will be performed by the Marin County coroner’s office.

“I’m shocked,” Diana Ware, stepmother of victim Barbara Ware, tells PEOPLE. “I won’t say I’m pleased he died but at the end there was justice for all the bad things he did in his life. We can now be at peace.”

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Image zoom Lonnie Franklin Jr. AP

In June of 2016, a Los Angeles jury decided that Franklin Jr. should be put to death on all counts for the murders of 10 women during a 23-year killing spree that began in 1984.

Jurors found Franklin guilty of the first-degree murders of Debra Jackson, Henrietta Wright, Mary Lowe, Bernita Sparks, Barbara Ware, Lachrica Jefferson, Monique Alexander, Princess Berthomieux, Valerie McCorvey and Janecia Peters.

He was also found guilty of the attempted murder of Enietra Washington, who testified that Franklin shot her, sexually assaulted her and took a Polaroid picture of her before pushing her out of his car 27 years ago.

Franklin’s victims were all young, vulnerable black women who lived in South Los Angeles and struggled with drug addiction. Their naked or partially clothed bodies were dumped in filthy neighborhood alleyways, left to rot under garbage and debris. They were shot at close range with a .25-caliber pistol, or strangled, or both.

Franklin’s last victim was 25-year-old Peters on Jan. 1, 2007. His youngest victim was Berthomieux, a 15-year-old runaway, who was found strangled in an alley in Inglewood, California, in 2002.

The Grim Sleeper cases remained cold for decades until the Los Angeles Police Department realized that seven of the killings in the ’80s were linked through DNA and ballistics to deaths in 2002, 2003 and 2007.

However, the killer’s DNA profile was not in CODIS, the national database for DNA. Franklin was finally caught in July 2010 through familial DNA testing after his then 28-year-old son, Christopher, was arrested for carrying a weapon in the summer of 2009 and had to give up a DNA swab.