Wash. Rev. Code § 80.04.460
Every public service company shall give immediate notice to the commission of every accident resulting in death or injury to any person occurring in its plant or system, in such manner as the commission may prescribe. Such notice shall not be admitted as evidence or used for any purpose against the company giving it in any action for damages growing out of any matter mentioned in the notice.
The commission may investigate any accident resulting in death or injury to any person occurring in connection with the plant or system of any public service company. Notice of the investigation shall be given in all cases for a sufficient length of time to enable the company affected to participate in the hearing and may be given orally or in writing, in such manner as the commission may prescribe.
Such witnesses may be examined as the commission deems necessary and proper to thoroughly ascertain the cause of the accident and fix the responsibility therefor. The examination and investigation may be conducted by an inspector or deputy inspector, and he or she may administer oaths, issue subpoenas, and compel the attendance of witnesses, and when the examination is conducted by an inspector or deputy inspector, he or she shall make a full and complete report thereof to the commission.
RCW 80.04.460