Governor Arnold Schwarzenegger rejected a recommendation to parole a member of Charles Manson's cult who was convicted of taking part in killings more than four decades ago.
Bruce Davis is serving life sentences for two 1969 deaths, although he was not involved in the infamous murders by Manson followers of actress Sharon Tate and six others in Los Angeles.
The California governor in a letter reversed a January decision by the state parole board, saying the murders were "especially heinous".
"I believe his release would pose an unreasonable risk of danger to society at this time," he wrote.
The board determined that Davis, 67, is ready to be released. While incarcerated, he earned a master's degree in religion and a doctorate in philosophy of religion, married and fathered a daughter.
Davis was convicted of helping kill musician Gary Hinman in his home and former stuntman Donald "Shorty" Shea, who lived in Manson's commune.
Mr Schwarzenegger acknowledged that Davis made "some credible gains" during his incarceration.
But in turning down the parole recommendation, he cited the "especially heinous" nature of the killings and Davis' repeated efforts to minimise his involvement.
The Republican governor also questioned whether Davis is still too willing to follow others' direction, noting his association with the American Nazi Party after he went to prison in 1972.
He also found troubling Davis' statements that he would be subservient to his wife should he be released. "Davis still exhibits conformist tendencies," Mr Schwarzenegger said in a decision he signed on June 22.