Pen. Code, § 904.6

(a) In any county or city and county, the presiding judge of the superior court, or the judge appointed by the presiding judge to supervise the grand jury, may, upon the request of the Attorney General or the district attorney or upon his or her own motion, order and direct the impanelment, of one additional grand jury pursuant to this section.

(b) The presiding judge or the judge appointed by the presiding judge to supervise the grand jury shall select persons, at random, from the list of trial jurors in civil and criminal cases and shall examine them to determine if they are competent to serve as grand jurors. When a sufficient number of competent persons have been selected, they shall constitute the additional grand jury.

(c) Any additional grand jury which is impaneled pursuant to this section may serve for a period of one year from the date of impanelment, but may be discharged at any time within the one-year period by order of the presiding judge or the judge appointed by the presiding judge to supervise the grand jury. In no event shall more than one additional grand jury be impaneled pursuant to this section at the same time.

(d) Whenever an additional grand jury is impaneled pursuant to this section, it may inquire into any matters which are subject to grand jury inquiry and shall have the sole and exclusive jurisdiction to return indictments, except for any matters which the regular grand jury is inquiring into at the time of its impanelment.

(e) It is the intent of the Legislature that all persons qualified for jury service shall have an equal opportunity to be considered for service as criminal grand jurors in the county in which they reside, and that they have an obligation to serve, when summoned for that purpose. All persons selected for the additional criminal grand jury shall be selected at random from a source or sources reasonably representative of a cross section of the population which is eligible for jury service in the county.