Attorney Lutzky is correct. Any checks received after a person dies should be deposited into an estate account. As Executrix, you should contact the Washington State Teachers Credit Union to inquire as to why there was no interest paid. If the answer isn't satisfactory and the foregone interest is significant, you should contact a litigation attorney in Washington. Good luck to you.
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After your mother died all funds should go to an estate account. Contact an attorney to assist you in establishing an estate account and either proceed through probate or administration if it was in NY. If in another state, then contact an attorney in that state.
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