ANAHEIM, Calif. — Two convicted sex offenders dutifully checked in with police every month and wore their GPS trackers around the clock — the rules of parole that are designed to tip off authorities if a freed felon backslides.

Yet for at least two months last fall, authorities claim, Franc Cano and Steven Dean Gordon were raping and killing at least four women — and probably a fifth — in the seedy prostitution hangouts of Orange County.