California Code, Government Code - GOV § 25250

At least biennially the board of supervisors shall examine and audit, or cause to be audited, the financial accounts and records of all officers having responsibility for the care, management, collection, or disbursement of money belonging to the county or money received or disbursed by them under authority of law. The audit shall encompass the immediately preceding two-year period, or any portion thereof not included in a prior audit. This financial examination or audit may be performed in coordination with the investigations conducted by the grand jury under Section 925 of the Penal Code, or the board of supervisors may resolve to accept reports delivered pursuant to Section 933 of the Penal Code in lieu of its own separate examination if such reports are found to fulfill some or all of the requirements of this section. In connection with the requirements of this section and Section 25253, the board of supervisors may employ the services of an independent certified public accountant or licensed public accountant to perform an examination of the financial statements in accordance with generally accepted auditing standards.