California Code, Penal Code - PEN § 939.6

(a) Subject to subdivision (b), in the investigation of a charge, the grand jury shall receive no other evidence than what is:

(b) Except as provided in subdivision (c), the grand jury shall not receive any evidence except that which would be admissible over objection at the trial of a criminal action, but the fact that evidence that would have been excluded at trial was received by the grand jury does not render the indictment void where sufficient competent evidence to support the indictment was received by the grand jury.

(c) Notwithstanding Section 1200 of the Evidence Code, as to the evidence relating to the foundation for admissibility into evidence of documents, exhibits, records, and other items of physical evidence, the evidence to support the indictment may be based in whole or in part upon the sworn testimony of a law enforcement officer relating the statement of a declarant made out of court and offered for the truth of the matter asserted. Any law enforcement officer testifying as to a hearsay statement pursuant to this subdivision shall have either five years of law enforcement experience or have completed a training course certified by the Commission on Peace Officer Standards and Training that includes training in the investigation and reporting of cases and testifying at preliminary hearings.