California Gov. Gavin Newsom reversed a recommendation to grant parole to Leslie Van Houten, who was convicted in the 1969 murder of grocer Leno LaBianca and his wife, Rosemary, at their Los Angeles home.
In a statement, Newsom wrote that Van Houten, 69, is still a threat, though she is considered by authorities to be a model prisoner and he has received reports of testimonials about her rehabilitation.
"While I commend Ms. Van Houten for her efforts at rehabilitation and acknowledge her youth at the time of the crimes, I am concerned about her role in these killings and her potential for future violence," Newsom wrote in a statement. "Ms. Van Houten was an eager participant in the killing of the LaBiancas and played a significant role."
Twice, the California parole board had previously recommended Van Houten be paroled, but former Gov. Jerry Brown rejected the board's recommendation both times.
"Nobody wants to put their name on her release, but when they're speaking honestly or off the record, everyone wants her to go home," Van Houten's attorney, Rich Pfeiffer, told the Associated Press.
Brown last denied Van Houten parole in January 2018, saying she still lays too much blame on Manson, who died in prison in November 2017, at age 83.
Van Houten was 19 years old when Manson ordered her and two other followers to kill the LaBiancas on Aug. 10, 1969.
During the grisly killings, the LaBiancas were stabbed multiple times, and the killers used their blood to scrawl on the couple's walls the words "Rise," "Death to Pigs" and "Helter Skelter" in an apparent reference to The Beatles' song of the same name.
The murders of Leno and Rosemary LaBianca came a day after other Manson followers killed "Valley of the Dolls" actress Sharon Tate, who was more than eight months pregnant at the time of her death. Tate, hairstylist Jay Sebring, heiress Abigail Folger, writer Wojciech Frykowski and teenager Steven Parent were killed at Tate's rental home in the secluded neighborhood of Benedict Canyon outside of Los Angeles on Aug. 9, 1969.
While Manson didn't commit the killings himself, he commanded others to do so. Prosecutors said he handed out knives and told his followers to commit savage murders of high-profile people around Los Angeles in a bid to start a race war in the United States. All seven victims were brutally stabbed.
Another former Manson follower Dianne Lake, now 66, who said she was with the Manson Family from ages 14 to 16, became a crucial witness for the prosecution during grand jury testimony. She said she was never involved with the killings, but recalled waking up to a "horrible smell" and seeing member Van Houten burning a purse and other items in the fireplace.
"She gave me a bag of coins, some kind of coin collection. It turns out that that belonged to Rosemary LaBianca," Lake told ABC News in a recent interview. "We took some acid and Leslie [Van Houten] and Susan Atkins and Patty Krenwinkle were telling me their participation in the Tate-LaBianca murders."
"Leslie [Van Houten], Susan [Atkins], Patty [Krenwinkel] and Tex [Watson] had all told me what they had done. And I was going to have to face Charlie. I was really afraid," she added.
Manson and his disciples -- Watson, Atkins, Krenwinkel and Van Houten -- were all convicted and given the death penalty, which was later commuted to life in prison.
During Van Houten's 2018 parole hearing, she told commissioners the remorse she feels for helping commit the murders is what has driven her to make amends in prison, where she has helped other inmates get their education and adjust to life in confinement.
"I think most of what I do is out of guilt for what I've done," Van Houten told the board at the time.
ABC News' Allie Yang contributed to this report.