California Code, Government Code - GOV § 12550

The Attorney General has direct supervision over the district attorneys of the several counties of the state and may require of them written reports as to the condition of public business entrusted to their charge.

When the Attorney General deems it advisable or necessary in the public interest, or when directed to do so by the Governor, the Attorney General shall assist any district attorney in the discharge of the district attorney's duties, and may, if deemed necessary, take full charge of any investigation or prosecution of violations of law of which the superior court has jurisdiction. In this respect the Attorney General has all the powers of a district attorney, including the power to issue or cause to be issued subpoenas or other process.