What is United States Commissioner?

Legal Definition
Each circuit court of the United States may appoint, in different parts of the district for which it is held, as many discreet persons as it may deem necessary, who shall be called "commissioners of the circuit court," and shall exercise the powers which are or may be conferred upon them. Rev. St. U. S. § 627 (In S. Comp. St. 1901, p. 499).
-- Black's Law Dictionary
Legal Definition
the officer of the federal district court who brings to court people accused of federal offenses.