Pursuant to Fed. R. Civ. P. 4(i)(1)(A), the United States and it agencies, corporations, officers, or employees, may be served by delivering "a copy of the summons and of the complaint to the United States attorney for the district where the action is brought—or to an assistant United States attorney or clerical employee whom the United States attorney designates in a writing filed with the court clerk."  The current designation filed with the Clerk can be found here.