The Secret Service was involved in the case of serial rapist Mike DeBardeleben, who would kidnap women, enslave, rape, and torture them. It started out as an investigation into the passing of counterfeit money, but agents discovered evidence that pointed to him engaging in horrific crimes. Interestingly, they had initially attempted to get assistance from the FBI, but, if I remember right, were told that there was not enough evidence to open a case. No victims at that point. FBI Files did a movie about it.