WASHINGTON (AP) _ The nation's prisons and jails held an estimated 283,800 mentally ill inmates in 1998, and they were more likely than other offenders to have committed violent offenses, the Justice Department reported Sunday.

The author of a department study cautioned that the results do not mean necessarily the mentally ill are more violent than others but that the violence figure could be skewed by other factors.

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

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 illnesses, and, to compensate for underreporting, those who had ever spent a night in a mental 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 violent offenses:

_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, compared with 26 percent of the others, had violent offenses.

_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, bureau statistician Paula Ditton.

Other causes could produce these figures, Ditton 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 that some mental patients released from hospitals to live in communities, part of a movement that began in the 1970s to shift patients from institutions, 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 females, 78 percent of the mentally ill had been abused compared to 51 percent of the others.

Ditton also noted that homelessness was more than double among mentally ill inmates than among others. For mentally ill inmates, 20 percent of those in state prisons and 30 percent of those in jails were homeless in the year before imprisonment, compared to 9 percent of the others in state prisons and 17 percent of the others in jails.

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.

Because the totals were based on self-reporting, they cannot be compared to mental illness rates in the general population, Ditton said.

But in National Institute of Mental Health studies in 1994 and 1998, a Northwestern University psychiatry professor, Linda Teplin, had interviewers independently assess the mental illnesses of Cook County, Ill., jail inmates. The inmates were found to have rates of mental illness two to four times higher than the general population, depending upon the type of disorder.

Teplin found 9 percent of male inmates and 18.5 percent of females had serious disorders, like schizophrenia, proportions she described as huge. She said the female figure was higher because ``only the most messed-up women end up in jail, the drug users and prostitutes,'' and criminal activity is more widespread among men.