English to Persian (Farsi) Law/Patents Law (general)

What is the most accurate Persian equivalent for "Federal Prosecutors"?
context:
Little headway was made, and in 1997, the local prosecutors handed the matter over to federal authorities so they could investigate possible civil rights violations. ... His persistent refusal to admit to having seen Cox effectively blocked the federal prosecutors’ attempt to indict the officers involved in the attack and no one was ever charged with the assault.