In Sacramento County, DeAngelo was charged with two counts of murder for the 1978 shootings of Kate and Brian Maggoire.
He also admitted to rape, burglary, and other crimes, said a report from BBC.
The cold-case DNA breakthrough leading to DeAngelo's arrest came shortly after best-selling true crime book "I'll Be Gone in the Dark" reinvigorated public interest in the infamous case.
Joseph James DeAngelo Jr. is appearing in a makeshift courtroom Monday - onstage in a ballroom at Cal State Sacramento - where he will plead guilty to the 13 counts of murder and 13 counts of kidnapping for purposes of robbery.
"The family members of murder victims have waited decades for justice", said Amy Holliday, Sacramento County's assistant chief deputy district attorney.
Resolving the case with a deal "at this time" would allow the remaining victims and families to "hear the defendant admit that he committed these acts and crimes". I didn't want to do those things.
In the gallery where the hearing took place, the chairs were spaced 10 feet apart in observance of social distancing.
Long-unsolved investigations into these serial killings in northern and southern California lasted decades. In the years preceding her death in April 2016, the true crime author thoroughly investigated the then-suspected crimes of the Golden State Killer, a moniker she established through her writing.
DeAngelo is believed to be the person behind more than a dozen murders, about 50 sexual assault cases, and ransacking of over 100 homes.
Initially, the crimes committed in the various regions were not linked together by the investigators. Officials only realized later on that the crimes were the work of only one man. Joseph James DeAngelo Jr. had remained nearly silent in court since his 2018 arrest until he repeatedly uttered the words "guilty" and "I admit" in a hushed and raspy voice as part of a plea agreement that will spare him the death penalty for a life sentence with no chance of parole. "The scope of Joseph DeAngelo's crimes is simply staggering". This is Santa Barbara District Attorney Joyce Dudley speaking about the significance of the admission of guilt to DeAngelo's victims.
DeAngelo had earlier never acknowledged any of the crimes including the killings. The website created a genetic profile leading them to a pool of relatives who all shared part of the killer's genetic material.
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