35 ILCS 615/8
For the purpose of administering and enforcing the provisions of this Act, the Department or any officer or employee of the Department designated, in writing, by the Director thereof, may hold investigations and, except for those matters reserved to the Illinois Independent Tax Tribunal, may hold hearings concerning any matters covered by this Act and may examine any books, papers, records or memoranda bearing upon the business transacted by any such taxpayer and may require the attendance of such taxpayer or any officer or employee of such taxpayer, or of any person having knowledge of such business, and may take testimony and require proof for its information. In the conduct of any investigation or hearing, neither the Department nor any officer or employee thereof shall be bound by the technical rules of evidence, and no informality in any proceeding, or in the manner of taking testimony, shall invalidate any order, decision, rule or regulation made or approved or confirmed by the Department. The Director or any officer or employee thereof shall have power to administer oaths to any such persons. The books, papers, records, and memoranda of the Department, or parts thereof, may be proved in any hearing, investigation or legal proceeding by a reproduced copy thereof under the certificate of the Director. Such reproduced copy shall, without further proof, be admitted into evidence before the Department or in any legal proceeding.
35 ILCS 615/8