SACRAMENTO (AP) - 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 a guard, 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 the freedom accorded peacekeepers lets them run drugs, order inmate assaults and commit other crimes....