Wash. Rev. Code § 54.24.120
All bonds, warrants, and revenue obligations issued under the authority of chapter 1, Laws of 1931 and chapter 182, Laws of 1941 shall be legal securities, which may be used by any bank or trust company for deposit with the state treasurer, or any county, city, or town treasurer, as security for deposits in lieu of a surety bond under any law relating to deposits of public moneys and shall constitute legal investments for trustees and other fiduciaries other than corporations doing a trust business in this state and for savings and loan associations, banks, and insurance companies doing business in this state. All such bonds, warrants, and revenue obligations and all coupons appertaining thereto shall be negotiable instruments within the meaning of and for all purposes of the negotiable instruments law of this state.
RCW 54.24.120
Investment securities: Article 62A.8 RCW.