WASHINGTON (AP) _ An estimated 283,800 mentally ill inmates were in U.S. prisons or jails in 1998, but more than four out of 10 received no treatment at all, the Justice Department says.

In the first comprehensive study of mental illness behind bars, the department's Bureau of Justice Statistics also estimated Sunday that an additional 547,800 mentally ill offenders have been released into the community on probation.

Even though a growing number of states and localities are under court order to provide mental health treatment for inmates, only 60 percent of those in state or federal prisons and 41 percent of those in jails received any form of mental health treatment during their sentence, including drugs, counseling or mental hospital stays.

Among probationers, 56 percent reported receiving some mental health treatment.

The researchers based the estimates on interviews with a random sample of state and federal prison inmates, jail inmates and probationers. They defined the mentally ill as those who reported their own illness and, to compensate for underreporting, those who had ever spent a night in a psychiatric hospital. They did not count anyone committed to a hospital after a verdict of not guilty by reason of insanity or anyone civilly committed to a mental hospital.

In all four groups studied, the mentally ill were more likely than other offenders to have committed a violent offense.

In state prisons, 53 percent of mentally ill inmates, compared with 46 percent of the other inmates, were locked up for violent crimes. Among federal inmates, 33 percent of the mentally ill but only 13 percent of other inmates had violent offenses.

Among local jail inmates, 30 percent of the mentally ill, against 26 percent of the others, had violent offenses. And among probationers, 28 percent of the mentally ill, but only 18 percent of the others, reported their current offense was violent.

``This does not mean that mentally ill offenders are more violent than other offenders,'' said the study's author, statistician Paula Ditton.

Other causes could produce these figures, she said: Police could have an easier time catching violent criminals who are mentally ill than those who are not; juries might be more willing to convict mentally ill defendants than others; or judges might be more inclined to sentence mentally ill violent offenders to prison than other violent offenders.

Indeed, a 1997 study in Pittsburgh, funded by the National Institute of Mental Health and the MacArthur Foundation, found that, among those without substance abuse problems, released mental patients are no more likely to engage in violence than other residents living in the same neighborhoods.

Substance abuse increased the level of violence among both groups, but the violence was significantly higher among recently released patients with substance abuse problems.

Ditton said she found the total of more than 830,000 mentally ill Americans either behind bars or on probation ``startlingly high.''

She said additional research would be needed to learn how the total got so large. She speculated, however, that some of the mental patients released from hospitals to the community in the de-institutionalization movement that began in the 1970s may have ended up behind bars.

More than twice the percentage of mentally ill inmates reported prior physical or sexual abuse than did other inmates _ among state inmates, 37 percent of the mentally ill and only 15 percent of the others were abused before going to prison. Among female inmates, 78 percent of the mentally ill had been abused, compared with 51 percent of others.

The estimates of mentally ill inmates and probationers as of mid-1998 included 16 percent of state prison inmates, local jail inmates and probationers and 7 percent of the much smaller total of federal prisoners.