In the U.S. federal government, regulatory and law enforcement agencies that investigate crimes must typically refer cases to the Department of Justice for prosecution at its discretion.
In a direct referral, agencies refer cases to the U.S. Attorney in the district where the crime occurred.
[1] The United States Congress and its members, in their investigative role, issue criminal referrals to the Justice Department as well.
[2] State attorneys general often refer federal crimes to the Justice Department.
Attorneys, such as the case against Michael Cohen in the Southern District of New York, which was referred by the Mueller investigation.