Iowa Code § 622.88: Subpoena issued.

If the officer is satisfied that the object is legal and proper, the officer shall issue a subpoena to bring the witness before the officer, and, if the witness fails then to make a full affidavit of the facts within the witness’ knowledge to the extent required of the witness by the officer, the latter may proceed to take the witness’ deposition by question and answer in the usual way, which may be used instead of an ordinary affidavit.