When a witness is not willing to testify or produce documents or other evidence voluntarily, the assistance of foreign authorities generally must be sought. The customary method of compelling evidence is by letter rogatory or “letter of request”, or if the country in question is a party to the Hague Evidence Convention, pursuant to those procedures.
b. Compulsion of Evidence via Letters Rogatory A letter rogatory, now more commonly known as a “letter of request”, is a formal request from a court in one country to “the appropriate judicial authorities” in all other country requesting compulsion of testimony or documentary or other evidence Most foreign countries will only accept letters rogatory from a court, and will not recognize a request from an administrative law judge. Letters rogatory can be used in civil and criminal matters, and have been used in administrative matters. The execution of a request for judicial assistance by the foreign court is based on comity between nations.