A new survey conducted by Yale law researchers and the Association of State Correctional Administrators (ASCA) found that more than 4,000 prisoners suffering from serious mental illness in the U.S. are being held in solitary confinement. Prisoners are held in isolation for 22 hours each day, for 15 days or more, despite the fact that long-term isolation can both trigger mental health issues, and exacerbate existing mental illness, the Guardian reported.

“They are basically being subjected to torture,” Amy Fettig, the Deputy Director of the ACLU National Prison Project told Fortune. Fettig explained that “serious mental illness” refers to the most vulnerable people, including those who are psychotic, or have schizophrenia or bipolar disorder.

Thirty-three states responded to the survey, with one third of them reporting that at least 10% of male prisoners with mental health issues were held in solitary confinement. In New Mexico specifically, institutions reported that 64% of mentally ill prisoners were being held in isolation.

Decades of research on solitary confinement has established the negative impacts of the practice on human beings generally. A 2014 study found that prisoners in New York City who were placed in solitary confinement were seven times more likely to harm or kill themselves. People suffering from serious mental illness are often catatonic, cut themselves and have suicide rates that are higher than any other part of the prison, according to Fettig.

The Yale and ASCA survey also found that nearly 2,000 prisoners have been held in isolation for more than six years, which the Guardian described as a “crisis point.” According to Fettig, the risks associated with long-term solitary confinement of prisoners with serious mental illness is “so well known and so well established that it’s a violation of the 8th amendment,” protects people from cruel and unusual punishment. She added that the number reported to the survey (4,000) is likely low, because mental health care and diagnosis in prison and jails is “notoriously bad.”

And while the total number of prisoners in isolation has gone down in recent years from 100,000 in 2014 to about 61,000 last year, the number has increased in 11 states, according to the Guardian.

“Part of the issue here is we have turned jails and prisons into mental health hospitals, and they aren’t mental health hospitals,” Fettig explained. Instead of treating people with mental illness, prisons place them in solitary to get rid of them. “They’re burying them alive in the prison system.”