39 CFR § 233.1 - Arrest and investigative powers of Postal Inspectors.
(a) Authorization. Postal Inspectors are authorized to perform the following functions in connection with their official duties:
(1) Serve warrants and subpoenas issued under the authority of the United States;
(2) Make arrests without warrant for offenses against the United States committed in their presence;
(3) Make arrests without warrant for felonies cognizable under the laws of the United States if they have reasonable grounds to believe that the person to be arrested has committed or is committing such a felony;
(4) Carry firearms; and
(5) Make seizures of property as provided by law.
(b) Limitations. The powers granted by paragraph (a) of this section shall be exercised only—
(1) In the enforcement of laws regarding property in the custody of the Postal Service, property of the Postal Service, the use of the mails, and other postal offenses. With the exception of enforcing laws related to the mails:
(i) The Office of Inspector General will investigate all allegations of violations of postal laws or misconduct by postal employees, including mail theft; and
(ii) The Inspection Service will investigate all allegations of violations of postal laws or misconduct by all other persons.
(2) To the extent authorized by the Attorney General pursuant to agreement between the Attorney General and the Postal Service, in the enforcement of other laws of the United States, if the Attorney General determines that the violation of such laws will have a detrimental effect upon the operations of the Postal Service.
(c) Administrative subpoenas may be served by delivering a copy to a person or by mailing a copy to his or her last known address. For the purposes of this provision, delivery of a copy includes handing it to the party or leaving it at the party's office or residence with a person of suitable age and discretion employed or residing therein. Service by mail is complete upon mailing.
(d) In conducting any investigation, Postal Inspectors are authorized to accept, maintain custody of, and deliver mail.