LOS ANGELES — The enduring mystery of why young people joined Charles Manson’s murderous family appeared to be at the heart of Gov. Jerry Brown’s decision Friday to reverse a parole board’s recommendation and keep Bruce Davis in prison.

Brown said he wants Davis, who has been behind bars for 42 years, to come clean about all the details of his involvement with Manson’s cult and the two gruesome killings of a stuntman and a musician.

It was the second time in less than three years that a California governor has rejected a parole board ruling in Davis’ case. Gov. Arnold Schwarzenegger refused his release in 2010, citing the heinous nature of Davis’ crimes and his efforts to minimize his involvement.

Brown repeated those reasons in his decision, but added his belief that Davis still has more to disclose about the killings.

“Until Davis can acknowledge and explain why he actively championed the Family’s interests and shed more light on the nature of his involvement, I am not prepared to release him,” Brown said.

The state parole board, citing the prisoner’s positive progress, approved release of the 70-year-old Davis, but the Democratic governor had the last word.

Brown gave his decision at the Los Angeles County courthouse after a meeting with District Attorney Jackie Lacey, who had recommended that Davis not be paroled.

Davis’ attorney, Michael Beckman, called the governor’s decision “horrible” and contrary to the findings of parole commissioners who conducted hearings for Davis and found him suitable for parole. He said Davis has told everything he knows.

“I have represented over 700 life prisoners, and of all of them, Bruce Davis is the most rehabilitated and qualified,” the attorney said.

He said Brown’s “paper review” of the case was insufficient to understand who Davis is today.

Brown said, “I find the evidence ... shows why he currently poses a danger to society if released from prison. Therefore, I reverse the decision to parole Mr. Davis.”

Beckman said the governor did not articulate any reason why Davis might pose a danger now.

If the problem is association with the Manson family, Beckman said, “They should pass a law saying if you were involved with these people you can’t get out. But there is no such law. “

Davis would have been only the second Manson-related murder defendant to be granted parole since the killing spree began in 1969.

Davis was not involved in the notorious Sharon Tate-LaBianca killings but was convicted with Manson and others in the murders of musician Gary Hinman and stuntman ranch hand Shorty Shea.

Manson was a direct participant in both killings, witnesses said.

Steve Grogan, another participant in those murders, was released in 1985 after he led police to where the bodies were buried.