28 CFR 600.1: Grounds for appointing a Special Counsel.

The Attorney General, or in cases in which the Attorney General is recused, the Acting Attorney General, will appoint a Special Counsel when he or she determines that criminal investigation of a person or matter is warranted and—

(a) That investigation or prosecution of that person or matter by a United States Attorney's Office or litigating Division of the Department of Justice would present a conflict of interest for the Department or other extraordinary circumstances; and

(b) That under the circumstances, it would be in the public interest to appoint an outside Special Counsel to assume responsibility for the matter.