220 ILCS 5/10-107
The Commission, each commissioner and each employee of the Commission properly authorized thereby shall have the right, at any and all times to inspect the papers, books, accounts and documents, plant, equipment or other property of any public utility, and the Commission, each commissioner and any administrative law judge of the Commission authorized to administer oaths shall have the power to examine under oath any officer, agent or employee of such public utility in relation to any matter within the jurisdiction of the Commission. A person other than a commissioner or administrative law judge demanding such inspection shall produce under the seal of the Commission his authority to make such inspection. A written record of the testimony or statement so given under oath shall be made and filed with the Commission. Information so obtained shall not be admitted in evidence or used in any proceeding except in proceedings provided for in this Act.
Any party to a proceeding before the Commission shall have the right to inspect the records of all hearings, investigations or inquiries conducted by or under the authority of the Commission, which may relate to the issues involved in such proceeding; and to submit suggestions as to other matters to be investigated or as to questions to be propounded. If the Commission is satisfied that such suggested investigation should be made or such suggested questions answered, and that the information desired is within the power of either party to furnish, it shall enter an order requiring the investigation to be made or the questions to be answered, and upon failure or refusal to comply with such order, the Commission shall either refuse to grant the relief prayed for by the party refusing to comply, or may grant the relief prayed for by the opposing party against the party refusing to comply.
220 ILCS 5/10-107