Cal. Veh. Code § 5001
The department may issue license plates for vehicles exempt from registration fees in the same series as plates issued for nonexempt vehicles. The plates may be issued for a one-year period and only upon the certification of the department that the issuance of the plates has been requested by the head of a criminal justice or a law enforcement agency of a city, county, or state or federal department, that the vehicle is assigned to persons responsible for investigating actual or suspected violations of the law or the supervision of persons liberated from a state prison or other institution under the jurisdiction of the Department of Corrections by parole or the supervision of persons liberated from an institution under the jurisdiction of the Department of the Youth Authority by parole, and is intended for use in the line of duty.
Ca. Veh. Code § 5001