Wash. Rev. Code § 81.04.510
Whether or not any person or corporation is conducting business requiring operating authority, or has performed or is performing any act requiring approval of the commission without securing such approval, shall be a question of fact to be determined by the commission. Whenever the commission believes that any person or corporation is engaged in operations without the necessary approval or authority required by any provision of this title, it may institute a special proceeding requiring such person or corporation to appear before the commission at a location convenient for witnesses and the production of evidence and bring with him or her or it books, records, accounts, and other memoranda, and give testimony under oath as to his or her or its operations or acts, and the burden shall rest upon such person or corporation of proving that his or her or its operations or acts are not subject to the provisions of this chapter. The commission may consider any and all facts that may indicate the true nature and extent of the operations or acts and may subpoena such witnesses and documents as it deems necessary.
After having made the investigation herein described, the commission is authorized and directed to issue the necessary order or orders declaring the operations or acts to be subject to, or not subject to, the provisions of this title. In the event the operations or acts are found to be subject to the provisions of this title, the commission is authorized and directed to issue cease and desist orders to all parties involved in the operations or acts.
In proceedings under this section, no person or corporation shall be excused from testifying or from producing any book, waybill, document, paper, or account before the commission when ordered to do so, on the ground that the testimony or evidence, book, waybill, document, paper, or account required of him or her or it may tend to incriminate him or her or it or subject him or her or it to penalty or forfeiture; but no person or corporation shall be prosecuted, punished, or subjected to any penalty or forfeiture for or on account of any account, transaction, matter, or thing concerning which he or she or it shall under oath have testified or produced documentary evidence in proceedings under this section: PROVIDED, That no person so testifying shall be exempt from prosecution or punishment for any perjury committed by him or her in his or her testimony.
RCW 81.04.510