His name is Steven, county officials tell us—the lean young man who sits alone on the floor, cross-legged and naked. We can't hear him; he's enclosed in a cell of thick plexiglass. But his lips move. He gestures vigorously and and talks as if to the ceiling.

"The other day he was banging on the windows and basically...having an outburst," says Sergeant Ron Riordan, our guide on a tour of the Hennepin County Jail in Minneapolis. Steven is "not somebody that could be out in any sort of general population environment because the other inmates would pick on him."

Steven also tried to hang himself. That explains his nakedness. "We restrict the inmate's clothing because he was tying it around his neck," Riordan explains.

At the Hennepin County Jail, as at most others in the country, large and small, coping with mental illness has become routine.

Sergeant Riordan oversees the intake process, where new inmates—51,000 a year, on average—are searched and signed in. "During that time [we] also ask the inmate a bunch of medical questions: Have any history of psychological problems? Ever tried to harm or kill yourself? Taken any psychiatric medications? Those types of things."

The jail has nurses on staff 24 hours a day. Psychologists visit several times a week. The special set of isolation cells where Steven is being held is for inmates who've behaved aggressively or "shown a potential to be highly suicidal," Riordan explains.

Steven was arrested for trespassing, we learn later. That's a charge that usually leads to little or no jail time. In Steven's case, his attorneys argue he's incompetent to stand trial, so he'll spend six weeks in jail waiting for a hearing. (Eventually he'll be declared incompetent and released.)

The Justice Department says mentally ill inmates generally serve longer sentences than those without mental disorders because they're more likely to break rules or get in fights.

As in early 19th-Century America, jails and prisons now function as the nation's asylums. The Justice Department estimates one in six inmates across the country has a severe mental illness. Most are locked up for relatively minor offenses. Many cycle in and out of jail repeatedly.

Next: Locked Up and Psychotic