Cal. Welf. & Inst. Code § 653

Whenever any person applies to the probation officer or the district attorney in accordance with subdivision (e) of Section 601.3, to commence proceedings in the juvenile court, the application shall be in the form of an affidavit alleging that there was or is within the county, or residing therein, a minor within the provisions of Section 601 and setting forth facts in support thereof. The probation officer or the district attorney, in consultation with the probation officer, shall immediately make any investigation he or she deems necessary to determine whether proceedings in the juvenile court should be commenced.