Colorado prison wardens can no longer send prisoners with major mental illness to solitary confinement, according to a memo distributed Thursday that solidifies state policy years in the making.

The numbers of mentally ill prisoners locked in cells alone for 23 hours each day have steadily dropped this fall: from 40 in September to fewer than 30 in November and now to just eight. The state wants the number at zero by the end of the year.

The memo from interim director of prisons Lou Archuleta to prison wardens said that “going forward,” staff members must not send prisoners with major mental illness to solitary confinement, also called administrative segregation.

“This is an enormous foundational step toward getting seriously mentally ill prisoners out of solitary confinement and into treatment,” said Rebecca Wallace, staff attorney at the American Civil Liberties Union of Colorado.

The Colorado Department of Corrections opened a residential treatment program for mentally ill prisoners at Centennial Correctional Facility in Cañon City in January, shutting down a prior program that treated mentally ill prisoners while they were held in solitary confinement.

But progress stalled when, two months later, corrections executive director Tom Clements was shot to death on his doorstep. The suspect: a parolee released directly from solitary confinement to the streets who was later killed by Texas deputies.

“We were moving along, and Tom was killed. We were at a standstill,” said Kellie Wasko, the department’s deputy executive director. After months with an interim executive director, and then the hiring of new director Rick Raemisch, “it was time to pick it back up and move on.”

Since this fall, the state has moved about 55 mentally ill prisoners into the residential treatment program. The 240-bed program had about 160 prisoners in September and now has about 215.

“As you know, we, as a department have been working to move all of our offenders housed in administrative segregations with major mental illness out of ad seg,” the memo said.

There were 662 prisoners in solitary confinement in September, down from more than 1,500 two years ago. Department officials on Wednesday announced a series of reforms intended to more closely monitor parolees released directly from solitary.

Department of Corrections officials did not announce the contents of the memo, but the ACLU, which has advocated against putting mentally ill prisoners in solitary cells, received a copy and shared it with media.

The ACLU’s Wallace called the policy a “very important first step.”

“There is still more important work to be done, but we want to take this moment to recognize something we have been asking the Department of Corrections to do for years,” she said.

Still, the ACLU is concerned the state’s definition of major mental illness is too narrow and will leave many prisoners with mental illness stuck in solitary confinement.

Also, the residential treatment program for mentally ill prisoners is too much like solitary confinement, Wallace said. One prisoner moved from solitary to the program spent an average of 12 minutes per week in individual therapy during the first six months of treatment, she said. The treatment program is understaffed, meaning prisoners will not get adequate help unless the state hires additional therapists, Wallace said.

“Is there money and the will to fill staff positions?” she asked.

Before transferring dozens of prisoners to the treatment program, the department had to evaluate thousands of inmates to determine whether they had “major” mental illness. About 1,500 — or 8 percent of the prison population — was determined to have major mental illness.

Not all prisoners with major mental illness will spend time in the residential treatment program. Many will live in general population if they can function there, Wasko said.

Jennifer Brown: 303-954-1593, jenbrown@denverpost.com or twitter.com/jbrowndpost