28 U.S.C. § 2711

A United States attorney or assistant United States attorney or a person authorized by the Attorney General— (1) upon his own affidavit or that of another credible person, stating the existence of either of the grounds of attachments enumerated in section 2710 of this title and (2) upon production of legal evidence of the debt may apply for a warrant of attachment to a judge, or, in his absence, to the clerk of any court of the United States having original jurisdiction of the cause of action.