Minn. Stat. § 388.151: UNMARKED VEHICLES; LICENSE PLATES.

Vehicles used by county attorneys to investigate allegations of criminal wrongdoings, to assist crime victims or witnesses, to aid in prosecuting criminal offenses, and for other uses consistent with the duties of the county attorney which the county attorney elects to operate as unmarked must be registered and must display passenger vehicle classification license number plates. The registrar of motor vehicles shall furnish the license plates at cost upon application and certification signed by the county attorney that the vehicles will be used exclusively for the purposes authorized by this section.