((a)) The district court of the district in which a person resides or is found may order him to give his testimony or statement or to produce a document or other thing for use in a proceeding in a foreign or international tribunal, including criminal investigations conducted before formal accusation. The order may be made pursuant to a letter rogatory issued, or request made, by a foreign or international tribunal or upon the application of any interested person and may direct that the testimony or statement be given, or the document or other thing be produced, before a person appointed by the court. By virtue of his appointment, the person appointed has power to administer any necessary oath and take the testimony or statement. The order may prescribe the practice and procedure, which may be in whole or part the practice and procedure of the foreign country or the international tribunal, for taking the testimony or statement or producing the document or other thing. To the extent that the order does not prescribe otherwise, the testimony or statement shall be taken, and the document or other thing produced, in accordance with the Federal Rules of Civil Procedure.

((b)) This chapter does not preclude a person within the United States from voluntarily giving his testimony or statement, or producing a document or other thing, for use in a proceeding in a foreign or international tribunal before any person and in any manner acceptable to him.