USC 28 § 547. Duties
Except as otherwise provided by law, each United States attorney, within his district, shall—
(1) prosecute for all offenses against the United States;
(2) prosecute or defend, for the Government, all civil actions, suits or proceedings in which the United States is concerned;
(3) appear in behalf of the defendants in all civil actions, suits or proceedings pending in his district against collectors, or other officers of the revenue or customs for any act done by them or for the recovery of any money exacted by or paid to these officers, and by them paid into the Treasury;
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