The law:

“3-2.120 Appointment of US Attorneys

United States Attorneys are appointed by the President with the advice and consent of the Senate for a four-year term. See 28 U.S.C. Sec. 541. Upon expiration of this term, the United States Attorney continues to perform the duties of the office until a successor is confirmed. United States Attorneys are subject to removal at the will of the President. See Parsons v. United States, 167 U.S. 314 (1897).