Inmates in the county jail in Williston, N.D., in July 2013. Andrew Burton/Getty Images

Allegations of rape and sexual assault involving inmates are increasing, and nearly half those assaults are committed against prisoners by correctional officers, according to a new report issued by the Justice Department’s Bureau of Justice Statistics (BJS).

Prison and jail administrators reported 8,763 cases of alleged sexual abuse of inmates 2011, representing an increase of 4 percent from the 8,404 that were reported in 2010 and an 11 percent jump from the 7,855 reported in 2009, the report said.

The report released late last week defined sexual victimization as any nonconsensual sexual acts, abusive touching, threats and verbal sexual harassment. It involved surveying federal and state prisons, private prisons, local jails, military prisons and jails in Indian country — 1.97 million inmates in total.

The issue of prison rape has received heightened attention since Congress passed the Prison Rape Elimination Act in 2003, which calls for prisons and jails to keep detailed records of incidents of rape, to be published by the government annually.

This year’s report, which crunched data from 2011, said that 10 percent of the cases reported that year were substantiated, meaning that they were confirmed to have happened after an investigation was launched.

That means 90 percent of the cases reported by inmates were not substantiated. The report did not clarify whether those cases were investigated and then dismissed.

Some 49 percent of the incidents that year involved prison staff members committing what the report called “sexual misconduct” or otherwise sexually harassing inmates, and the remaining 51 percent involved inmates assaulting fellow inmates.

Among the substantiated staff-on-inmate cases in 2011, 54 percent were committed by women, the report said. From 2009 to 2011, 84 percent of the substantiated staff-on-inmate cases involved a sexual relationship with a female staff member that “appeared to be willing,” compared with 37 percent of the cases involving male staff members during the same period. The report noted, however, that regardless of whether the sexual relationship between an inmate and a correctional officer was consensual, it is illegal.

In the cases of sexual assault or “willing” sexual relationships with staff members, more than three-quarters of the correctional officers resigned or were fired and just 45 percent were arrested or prosecuted.

Women prisoners appeared to experience disproportionate numbers of sexual assaults; while they represented 7 percent of state and federal prison inmates from 2009 to 2001, 33 percent of staff-on-inmate and 22 percent of inmate-on-inmate cases involved female inmates.

Two-thirds of the inmates who had been sexually assaulted by other inmates received medical examinations, and one-third were given rape kits.

The report did not indicate whether the increased incidence of alleged rapes and sexual assaults in prisons and jails might have been due to more reporting by inmates or to heightened awareness of the problem by prison staff.

BJS statistician Allan Beck, who was a co-author of the report, told Reuters that a study from May 2013 (PDF) conducted by the same agency came up with much larger numbers, tallying some 80,000 inmate allegations of sexual abuse or assault during 2011 and 2012.

“Of course, we find much higher rates of sexual victimization through inmates’ self-reports than what comes through in the official records,” he told Reuters.