39 C.F.R. § 959.18 Subpoenas.
Title 39 - Postal Service
The Postal Service is not authorized by law to issue subpoenas requiring the attendance or testimony of witnesses or the production of documents. This does not affect the authority of the Chief Postal Inspector to issue subpoenas for the production of documents or information pursuant to §233.1(c) of this chapter. [39 FR 33213, Sept. 16, 1974, as amended at 56 FR 55825, Oct. 30, 1991]
Title 39: Postal Service
PART 959—RULES OF PRACTICE IN PROCEEDINGS RELATIVE TO THE PRIVATE EXPRESS STATUTES
§ 959.18 Subpoenas.

