Federal Rules of Civil Procedure (FRCP)
4(i)(1) requires:
(A) that “a copy of the summons and of the complaint [be delivered] to the United States attorney for the district where the action in brought…”
(B) that “a copy of each [summons and the complaint be sent] by registered or certified mail to the Attorney General of the United States at Washington, D.C.; and”
4(i)(2) provides: “To serve a United States agency or corporation, or a United States officer or employee sued only in an official capacity, a party must serve the United States and also send a copy of the summons and of the complaint by registered or certified mail to the agency, corporation, officer, or employee.”
Beware of some devious Court Clerks - an article on LinkedIn by Chungsoo Lee.