Wash. Rev. Code § 35.58.510
All banks, trust companies, bankers, savings banks, and institutions, building and loan associations, savings and loan associations, investment companies and other persons carrying on a banking or investment business, all insurance companies, insurance associations, and other persons carrying on an insurance business, and all executors, administrators, curators, trustees and other fiduciaries, may legally invest any sinking funds, moneys, or other funds belonging to them or within their control in any bonds or other obligations issued by a metropolitan municipal corporation pursuant to this chapter. Such bonds and other obligations shall be authorized security for all public deposits. It is the purpose of this section to authorize any persons, political subdivisions and officers, public or private, to use any funds owned or controlled by them for the purchase of any such bonds or other obligations. Nothing contained in this section with regard to legal investments shall be construed as relieving any person of any duty of exercising reasonable care in selecting securities.
RCW 35.58.510