Susan Atkins, who committed one of modern history's most notorious crimes when she joined Charles Manson and his gang for a string of killings in 1969 that terrorized Los Angeles and put her in prison for the rest of her life, has died. She was 61.

Atkins was diagnosed in 2008 with brain cancer and was receiving medical treatment at the Central California Women's Facility in Chowchilla, where she died at 11:46 p.m. Thursday, said Terry Thornton, a spokeswoman for the state Department of Corrections and Rehabilitation.

Thornton attributed the death to natural causes, but an official cause of death will be determined by the Madera County coroner's office after a review of Atkins' medical history.

Convicted of eight murders, Atkins served more than 38 years of a life sentence at the California Institution for Women in Chino. She was the longest-serving prisoner among women currently held in the state's penitentiaries, Thornton said. That distinction now falls to Patricia Krenwinkel, who was convicted along with Atkins in the Tate-LaBianca murders

Although prison staffers and clergy workers commended Atkins' behavior during her many years behind bars, she was repeatedly denied parole, with officials citing the cruel and callous nature of her crimes.

In June 2008, she appealed to prison and parole officials for compassionate release, but the state parole board denied the request. On Sept. 2, she was wheeled into her last parole hearing on a hospital gurney, but was turned down by a unanimous vote of the 12-member California Board of Parole.

Atkins confessed to killing actress Sharon Tate -- the pregnant wife of director Roman Polanski -- who was stabbed 16 times and hanged; Tate's nearly full-term fetus died with her. The next night, Atkins accompanied Manson and his followers when they broke into the Los Feliz home of Leno and Rosemary LaBianca and killed them.

"She was the scariest of the Manson girls," said Stephen Kay, a former Los Angeles County deputy district attorney who helped prosecute the case and argued against Atkins' release at her parole hearings. "She was very violent."

Former chief prosecutor Vincent Bugliosi, who sought and won death sentences for Atkins, Manson and other followers, said Atkins would be remembered "obviously as a member of a group that committed among the most horrendous crimes in American history. She apparently made every effort to rehabilitate herself."

He added: "It has to be said that she did pay substantially, though not completely, for her incredibly brutal crimes. And to her credit, she did renounce -- and, I believe, sincerely -- Charles Manson."

It was Atkins who broke open the case when she bragged of her participation in the slayings to cellmates at Sybil Brand Institute in East Los Angeles, where she was being held on other charges; two of her cellmates told authorities of her confession.

Atkins subsequently appeared before a grand jury, providing information that led to her own indictment, as well as that of Manson and others. Later, in a lurid, 10-month trial, she provided crucial testimony that fed the public's fascination with Hollywood celebrities, drugs, sex and violence.

It also left an unshakable image of Atkins as a remorseless killer, who taunted the court at her sentencing with chilling words: "You'd best lock your doors," she said, "and watch your own kids."

In 1971, two separate juries found Manson, Atkins, Krenwinkel and Charles "Tex" Watson guilty on seven counts of first-degree murder. Another Manson follower, Leslie Van Houten, was convicted of two murders.

All received the death sentence, later reduced to life terms after the California Supreme Court abolished the death penalty in 1972. (The Legislature later reenacted the death penalty statute.) Manson, Krenwinkel, Watson and Van Houten remain in prison.

Atkins also pleaded guilty to the murder of musician Gary Alan Hinman, who was killed in a dispute over money shortly before the Tate-LaBianca murders. She received another life sentence for the Hinman killing.

In prison, Atkins embraced Christianity and apologized for her role in the crimes. Prison staff endorsed her release at a hearing in 2005, but she was denied parole for the 13th time.

Born Susan Denise Atkins in San Gabriel on May 7, 1948, she grew up in San Jose, the middle child of three. When she was 15, her mother died of cancer. Her father sold the family home and all their furnishings to pay the hospital bills. Atkins began failing school and her father became an alcoholic who frequently left Susan and her younger brother, Steven, to fend for themselves.

Her father eventually abandoned them for good. Susan and her brother moved to the rural Cental Valley town of Los Banos, where their grandparents lived. Susan enrolled in high school and got a job as a waitress but was overwhelmed by the stress of trying to care for her brother, work and go to class. At one point, she and Steven were in foster care. Susan dropped out of school in the 11th grade and started drifting. Years later, she would describe her frame of mind during this period as "extremely angry, extremely vulnerable and directionless."

Susan Atkins looks back as Charles Manson arrives in court in 1970. She was convicted on seven counts of first-degree murder in the Tate-LaBianca killings, including the death of pregnant actress Sharon Tate. (Associated Press / March 26, 2013)