RCW 32.20.110: District bonds secured by taxing power.

A mutual savings bank may invest its funds in the bonds of any port district, sanitary district, water-sewer district, tunnel district, bridge district, flood control district, park district, or highway district in the United States which has a population as shown by the last decennial federal census of not less than one hundred fifty thousand inhabitants, and has taxable real property with an assessed valuation in excess of two hundred million dollars and has power to levy taxes on the taxable real property therein for the payment of the bonds without limitation of rate or amount.