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It is correct that the power of attorney expired at your mother's death. In order to subpoena documents, commonly referred to as a "subpoena duces tecum", you must have an open lawsuit because the clerk of the court actually issues the subpoena. If there is a pending lawsuit, it's easy. If not, you cannot "subpoena" the records. However, the personal representative of your mother's estate (an executor, for example) can request the documents with proof of letters testimentary or other forms of authority.
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