If, after his delivery to the warden for execution, there is good reason to believe that a defendant, under judgment of death, has become insane, the warden must call such fact to the attention of the district attorney of the county in which the prison is situated, whose duty it is to immediately file in the superior court of such county a petition, stating the conviction and judgment, and the fact that the defendant is believed to be insane, and asking that the question of his sanity be inquired into. Thereupon the court must at once cause to be summoned and impaneled, from the regular jury list of the county, a jury of 12 persons to hear such inquiry.