(Reuters) - Nearly 9,000 incidents of sexual victimization against inmates in U.S. prisons and jails were reported in 2011, with roughly half of them involving corrections staff, according to a report by the federal Bureau of Justice Statistics.
The number of incidents showed a "significant increase" over about 8,400 incidents reported in 2010 and 7,855 reported in 2009, it said. The year 2011 was the most recent cited in the bureau study, published on Thursday.
The number has been rising in each of the last five years, said the study, which defined sexual victimization as including non-consensual acts, abusive touching, threats, indecent exposure and harassment.
Of the 8,763 such incidents reported in 2011, 10 percent were substantiated, the bureau said. The study looked at incidents reported by correctional administrators.
About half of all the allegations involved staff directed toward inmates, it said, and half involved inmates with other inmates.
Another study released in May by the bureau showed some 80,000 victim allegations among inmates, said Allan Beck, a BJS statistician and an author of the report.
"Of course we find much higher rates of sexual victimization through inmates' self-reports than what comes through in the official records," he said.
But the nature of the incidents has similarities, he said.
"It's a matter of scale," Beck said.
Any sexual contact between inmates and staff is illegal, the bureau noted.
More than three-quarters of staff perpetrators resigned or were fired, and nearly half were arrested, prosecuted or convicted, the study found.
The findings were based on annual surveys of adult prisons and jails throughout the nation, it said. The data for 2011 was collected from facilities holding 1.97 million inmates.
The surveys have been conducted yearly since 2004 in compliance with the federal Prison Rape Elimination Act.
(Reporting by Ellen Wulfhorst; editing by Gunna Dickson)