A survey by the corrections agency in 2010 found that 22 percent of the 26,700 inmates in state prisons were diagnosed mentally ill, a figure that is in line with national estimates by the U.S. Department of Justice.
Advocates say the number of mentally ill people in prison has risen over the years as states have cut budgets for treatment.
"If you have people on the outside who are not getting adequate treatment and they engage in criminal behavior, they'll end up in the DOC," ACLU Legal Director Ken Falk said at a news conference Wednesday.
Josh Sprunger, executive director for the Indiana chapter of the National Alliance on Mental Illness, said the state corrections agency has improved its treatment of mentally ill inmates in the last decade, but more resources are needed to get people into treatment in the first place.
"They don't belong in DOC," he said. "We could really save the state a lot of resources if we could get people in recovery before charges are even filed."
Pratt found that mental health treatment in the prison system "is principally limited to issuance of medication, prisoner conversations with the mental health staff, and mental health staff's response to incidents of actual and attempted self-harm."
"The pervasive function of mental health staff within the IDOC has become a mixture of responding to crises and responding to prisoner requests to be seen," she added in her 37-page opinion issued in federal court in Indianapolis more than a year after a July 2011 bench trial.
Pratt noted one instance in which a prisoner committed suicide about a week after he missed two mental health appointments — at least one because no guards were available to escort him.