Wash. Rev. Code § 81.04.090
The commission may, in its discretion, prescribe the forms of any and all accounts, records and memoranda to be kept by public service companies, including the accounts, records and memoranda of the movement of traffic, sales of its product, the receipts and expenditures of money. The commission shall at all times have access to all accounts, records and memoranda kept by public service companies, and may employ special agents or examiners, who shall have power to administer oaths and authority, under the order of the commission, to examine witnesses and to inspect and examine any and all accounts, records and memoranda kept by such companies. The commission may, in its discretion, prescribe the forms of any and all reports, accounts, records and memoranda to be furnished and kept by any public service company whose line or lines extend beyond the limits of this state, which are operated partly within and partly without the state, so that the same shall show any information required by the commission concerning the traffic movement, receipts and expenditures appertaining to those parts of the line within the state.
RCW 81.04.090