Sirhan was recommended for parole in August, after spending 53 years in prison for the 1968 killing. Two of Kennedy's sons, Robert F. Kennedy, Jr. and Douglas Kennedy, supported the release during Sirhan's 16th appearance before the California Parole Board. But other family members, including RFK's widow Ethel, felt Sirhan should remain imprisoned.
Newsom decided to reverse the parole board's decision after determining that Sirhan "currently poses an unreasonable threat to public safety," a statement from the governor's office said.
"The Governor reached his decision based on several factors, including Mr. Sirhan's refusal to accept responsibility for his crime, lack of insight and accountability required to support his safe release, failure to disclaim violence committed in his name, and failure to mitigate his risk factors," the statement said.
In a Los Angeles Times Op-Ed explaining his decision, Newsom wrote that Sirhan had recorded his plans to kill Kennedy prior to the assassination and that decades after the killing, "Sirhan began dodging responsibility" and also recently dismissed the "relevance of his status as an ideological lightning rod."
Newsom added that Sirhan, now 77, "remains a potent symbol of political violence."
"He does not understand, let alone have the skills to manage, the complex risks of his self-created notoriety. He cannot be safely released from prison because he has not mitigated his risk of fomenting further political violence," the governor wrote.
Through the review process, Newsom noted his penchant for Kennedy, telling reporters he keeps a framed photo of the former senator at the entrance of his office.
CNN has reached out to Sirhan's attorney for comment.
Kennedy family members who opposed Sirhan's release said in a statement they were "deeply relieved" by the governor's decision.
"The killer's violent act contradicted the values of openness, dialogue, and democratic change that Robert Kennedy embraced and that underlie our political system," they said. "The offender must transform himself."
"Because of how entwined into popular culture this murder has become, amplified by the regularity of the inmate's attempts to be freed, our family has been forced to watch our husband and father be killed thousands of times," they said, adding that the governor's decision "represents the vindication of the rule of law over all who would betray it with hatred and violence."
"We are deeply grateful for this decision, aimed at ensuring that no family nor our nation will suffer the same heart-breaking, irredeemable loss," they added.
Sirhan -- who was 24 at the time -- shot Kennedy in the kitchen of the Ambassador Hotel in Los Angeles following a campaign event in which Kennedy celebrated primary victories in his run for the Democratic nomination for president in 1968.
An attorney for Sirhan previously described him as a Palestinian who became a refugee at 4 years old and "witnessed atrocities" before emigrating to the US as a teenager.
Originally sentenced to death, Sirhan's sentence was commuted to life in prison in 1972 after the California State Supreme Court declared the death penalty unconstitutional.
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