California's troubled prisons are being investigated anew, this time for relying on violent felons to help keep order behind bars.The practice has led to two slayings in just over one year. The Associated Press has found that the killings of a guard and an inmate involved so-called ``peacekeepers.'' These are the high-ranking gang members and other influential prisoners entrusted with controlling fellow inmates, smoothing racial tension and cooperating with staff. The practice has drawn previously unreported scrutiny from federal investigators, who are probing a peacekeeper's role in the Sacramento-area shooting of a prisoner by a guard. Prison system investigators have...