The California prison system's use of some of its toughest, most feared inmates to help keep order behind bars led to the slaying of an officer, state investigators say. And the FBI is looking into whether the practice contributed to a second killing.

Although the practice is banned in some states, California's top corrections official defends the limited use of "peacekeepers." These influential inmates are entrusted to help the staff, smooth racial tension and in some cases control fellow prisoners. Critics worry that the freedom accorded peacekeepers lets them run drugs, order inmate assaults and commit other crimes. Now the practice has come under scrutiny following two California slayings in which high-ranking gang members serving as peacemakers are alleged to have played a role.

Last January, a peacekeeper who had been released from his cell to mediate following a race riot stabbed an officer to death in Chino, said Brett Morgan, chief deputy for the prison system's inspector general. Just weeks before, a peacekeeper at a Sacramento-area prison ordered an assault that ended with an officer killing an inmate, according to confidential Corrections Department reports obtained by The Associated Press.