18 CFR § 1b.14 - Subpoenas.
(a) Service of a subpoena upon a person named therein shall be made by the investigating officer (1) by personal delivery, (2) by certified mail, (3) by leaving a copy thereof at the principle office or place of business of the person to be served, (4) or by delivery to any person designated as agent for service or the person's attorney.
(b) At the time for producing documents subpoenaed in an investigation, the subpoenaed party shall submit a statement stating that, if true, such person has made a diligent search for the subpoenaed documents and is producing all the documents called for by the subpoena. If any subpoenaed document(s) are not produced for any reason, the subpoenaed party shall state the reason therefor.
(c) If any subpoenaed documents in an investigation are withheld because of a claim of the attorney-client privilege, the subpoenaed party shall submit a list of such documents which shall, for each document, identify the attorney involved, the client involved, the date of the document, the person(s) shown on the document to have prepared and/or sent the document, and the person(s) shown on the document to have received copies of the document.