The commissioner has power to require any person subject to the taxes imposed by this chapter to furnish any additional information deemed by the commissioner to be necessary for the purpose of correctly computing the amount of the tax, and to examine the books, records, and files of such person, and has power to conduct hearings and compel the attendance of witnesses, the production of books, records, and papers of any person, and full authority to make any investigation or hold any inquest deemed necessary to a full and complete disclosure    of the true facts as to the amount of production or generation from any coal conversion plant, and as to the rendition thereof for taxing purposes.