Hundreds of prisoners housed in the California penitentiary system will be released from solitary confinement, where dozens of inmates have been detained for more than a decade, under the details of a settlement announced Tuesday. The agreement raises hopes among prison reform advocates for a national shift in the use of solitary confinement.

Signs of that shift are mounting, with recent reforms to solitary confinement practices in states ranging from Maine to Illinois to New York. In addition, earlier this year, a Louisiana judge released Albert Woodfox from prison after he spent 40 years in solitary, making national headlines. And Justice Anthony Kennedy, the key swing vote on the United States Supreme Court, raised the issue of solitary confinement in concurring with a ruling on a different subject earlier this year.

“Research still confirms what this Court suggested over a century ago: Years on end of near-total isolation exacts a terrible price,” he wrote, referencing the 1890 court decision In re Medley.

The California settlement ends a lawsuit filed three years ago by the long-term prisoners in solitary confinement at the maximum-security Pelican Bay State Prison in northern California. They were detained for 23 hours a day in so-called Secure Housing Units (SHUs) without any access to human contact or the outside world. The one hour when prisoners could leave their concrete-walled cell was set aside for exercise, which they perform in a cage.

The lawsuit challenged the use of solitary confinement to control prison gangs, alleging it constituted cruel and unusual punishment in violation of the Eighth Amendment.

"This settlement represents a monumental victory for prisoners and an important step toward our goal of ending solitary confinement in California, and across the country," said the lawsuit’s plaintiffs in a written statement. "The prisoners’ human rights movement is awakening the conscience of the nation to recognize that we are fellow human beings."

The state’s correctional officer union – which opposed the settlement – defended what they saw as the legitimate use of solitary confinement to prevent prison violence.

"In our view, California will return to the prison environment of the '70s and '80s, when inmate-on-inmate homicides were at the highest levels and staff were killed," Nichol Gomez-Pryde, spokeswoman for the California Correctional Peace Officers Association, told the Los Angeles Times.

During the case, a panel of experts told state corrections officials that high numbers of inmates in lengthy isolation did little to improve prison security and could lead to serious mental health issues.

Lawsuits against isolation units are expected to increase given Justice Kennedy’s language on the issue, which seemed to invite legal challenges.

"I've never seen it happen before," Amy Fettig, an attorney with the American Civil Liberties Union's National Prison Project, told The Associated Press. "Every civil rights attorney in the country heard his call. Certainly, Justice Kennedy has opened the door and we're all preparing to walk through it."

California’s practice of segregating a large number of high-risk inmates in isolation units grew from concerns about gang-related violence, which peaked in the 1970s and '80s. Under the settlement, solitary confinement will be limited to prisoners who commit felonies behind bars instead of simply those with gang affiliations.

According to numbers from the corrections department, the state currently has about 6,400 prisoners in isolation units, with about 1,500 expected to be released from solitary confinement due to the settlement.

All inmates who were sent to solitary because of gang affiliations will have their cases reviewed within a year, with a high likelihood of release from isolation units if they did not violate any rules. Prisoners who have been held in solitary for at least 10 years are expected to be released almost immediately.

While the majority will be transitioned back into the general prison population, high-risk prisoners will now be housed in small, high-security units that will group dangerous inmates together and provide access to privileges like visitation, phone calls, and library access.

Long-term solitary confinement has been characterized as abuse and torture by human rights groups such as Amnesty International and the American Civil Liberties Union and has been tied to severe psychological problems, including hallucinations and suicidal thoughts.

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The plight of prisoners in solitary confinement was highlighted through actions of civil disobedience, including a statewide hunger strike in 2013 led by four rival prison gang leaders.

California Corrections Secretary Jeffrey Beard said the state prison system, which is already grappling with issues of overcrowding, could not make a compelling case for keeping the prisoners in isolation units. Secretary Beard pointed to largely successful efforts by the state during the past two years to reduce the number of prisoners in isolation due to the state shifting criteria for behavior that is considered gang activity.