The claims of the Indians of California under the provisions of this subchapter shall be presented by petition, which shall be filed within three years after May 18, 1928. Said petition shall be subject to amendment. The petition shall be signed and verified by the attorney general of the State of California. Verification may be upon information and belief as to the facts alleged. Official letters, papers, documents, and public records, or certified copies thereof, may be used in evidence and the departments of the Government shall give the said attorney access to such papers, correspondence, or furnish such certified copies of record as may be necessary in the premises free of cost.