15 U.S.C. § 15f

(a) Notification to State attorney general Whenever the Attorney General of the United States has brought an action under the antitrust laws, and he has reason to believe that any State attorney general would be entitled to bring an action under this Act based substantially on the same alleged violation of the antitrust laws, he shall promptly give written notification thereof to such State attorney general. (b) Availability of files and other materials To assist a State attorney general in evaluating the notice or in bringing any action under this Act, the Attorney General of the United States shall, upon request by such State attorney general, make available to him, to the extent permitted by law, any investigative files or other materials which are or may be relevant or material to the actual or potential cause of action under this Act.