California Code, Labor Code - LAB § 5602

No writ of attachment shall be issued except upon the order of the appeals board. Such order shall not be made where it appears from the application or affidavit in support thereof that the employer was, at the time of the injury to the employee, insured against liability imposed by this division by any insurer. If, at any time after the levying of an attachment, it appears that such employer was so insured, and the requisites for dismissing the employer from the proceeding and substituting the insurer as defendant under any method prescribed by this division are established, the appeals board shall forthwith discharge the attachment.