Charles Manson devotee Bruce Davis was denied parole by California Gov. Arnold Schwarzenegger.
The governor in a letter made public on Monday 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.