Gov. Code, § 25170

Whenever the board of supervisors deems it necessary or important to examine any person as a witness upon any subject or matter within the jurisdiction of the board, or to examine any officer of the county in relation to the discharge of his official duties as to the receipt or disposition by him of any money, or concerning the possession or disbursement by him of any property belonging to the county, or to use, inspect, or examine any books, account, voucher, or document in the possession or under the control of the person or officer relating to the affairs or interests of the county, the chairman of the board shall issue a subpena, in proper form, commanding the person or officer to appear before it, at a time and place therein specified, to be examined as a witness. The subpena may require the person or officer to produce all books, papers, and documents in his possession or under his control, relating to the affairs or interests of the county.