California Code, Penal Code - PEN § 904.4

(a) In any county having a population of more than 370,000 but less than 400,000 as established by Section 28020 of the Government Code, the presiding judge of the superior court, upon application by the district attorney, may order and direct the drawing and impanelment at any time of one additional grand jury.

(b) The presiding judge may 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.  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 that are subject to grand jury inquiry and shall have the sole and exclusive jurisdiction to return indictments, except for any matters that the regular grand jury is inquiring into at the time of its impanelment.

(e) If an additional grand jury is also authorized by another section, the county may impanel the additional grand jury authorized by this section, or by the other section, but not both.