Wash. Rev. Code § 33.24.070
An association may invest its funds in the revenue bonds of any city, town, district, or political subdivision of this state for the payment of which revenue of the city, town, district or political subdivision utility or revenue producing facility is irrevocably pledged.
It may invest its funds in the light, water, or sewer revenue bonds of any city or other municipal corporation in the United States having a population of not less than fifty thousand inhabitants as determined by the last federal census, which has not defaulted in the payment of interest or principal upon this or any like obligation, including those for which its credit was pledged, within ten years last past, for the payment of which the entire revenue of the city's or other municipal corporation's light, water, or sewer system, less maintenance and operating costs, is irrevocably pledged.
The aggregate of the investments of an association in any issue of such revenue bonds shall at no time exceed five percent of the amount of its savings accounts.
RCW 33.24.070