49 C.F.R. § 1.73
The Inspector General shall report to and be under the general supervision of the Secretary and Deputy Secretary. The Inspector General has such authority as is provided by the Inspector General Act of 1978, as amended, and as is otherwise provided by law. Authorities provided to the Inspector General by law are reserved to the Inspector General. In accordance with the statutory intent of the Inspector General Act to create an independent and objective unit, the Inspector General is authorized to make such investigations and reports relating to the administration of the programs and operations of the Department as are, in the judgment of the Inspector General, necessary and desirable. Neither the Secretary nor the Deputy Secretary shall prevent or prohibit the Inspector General from initiating, carrying out, or completing any audit or investigation, or from issuing any subpoena during the course of any audit or investigation.
49 C.F.R. §1.73