49 CFR § 105.55 - Refusal to obey a subpoena.
(a) Quashing or modifying a subpoena. If you receive a subpoena, you can ask PHMSA to overturn (“quash”) or modify the subpoena within 10 days after the subpoena is served on you. Your request must briefly explain the reasons you are asking for the subpoena to be quashed or modified. PHMSA may then do the following:
(1) Deny your request.
(2) Quash or modify the subpoena.
(3) Grant your request on the condition that you satisfy certain specified requirements.
(b) Failure to obey. If you disobey a subpoena, PHMSA may ask the Attorney General to seek help from the United States District Court for the appropriate District to compel you, after notice, to appear before PHMSA and give testimony, produce subpoenaed documents or physical evidence, or both.