Proceedings before an agency of the U.S. refers to any proceeding before the agency with respect to which the agency is authorized to issue subpoenas, take testimony or receive other information from witnesses under oath.
According to 18 USCS § 1505, it is an offense to obstruct proceedings before departments, agencies, and committees.
18 USCS § 1505 reads as follows:
§ 1505. Obstruction of proceedings before departments, agencies, and committees
Whoever, with intent to avoid, evade, prevent, or obstruct compliance, in whole or in part, with any civil investigative demand duly and properly made under the Antitrust Civil Process Act [15 USCS §§ 1311 et seq.], willfully withholds, misrepresents, removes from any place, conceals, covers up, destroys, mutilates, alters, or by other means falsifies any documentary material, answers to written interrogatories, or oral testimony, which is the subject of such demand; or attempts to do so or solicits another to do so; or
Whoever corruptly, or by threats or force, or by any threatening letter or communication influences, obstructs, or impedes or endeavors to influence, obstruct, or impede the due and proper administration of the law under which any pending proceeding is being had before any department or agency of the United States, or the due and proper exercise of the power of inquiry under which any inquiry or investigation is being had by either House, or any committee of either House or any joint committee of the Congress--
Shall be fined under this title, imprisoned not more than 5 years or, if the offense involves international or domestic terrorism (as defined in section 2331 [18 USCS § 2331]), imprisoned not more than 8 years, or both.