California Code, Penal Code - PEN § 939.2

A subpoena requiring the attendance of a witness before the grand jury may be signed and issued by the district attorney, his investigator or, upon request of the grand jury, by any judge of the superior court, for witnesses in the state, in support of the prosecution, for those witnesses whose testimony, in his opinion is material in an investigation before the grand jury, and for such other witnesses as the grand jury, upon an investigation pending before them, may direct.