Iowa Code § 633.297: Witnesses unavailable.

If all of such witnesses are deceased or otherwise not available, then it shall be permissible to prove said will by the sworn testimony of two credible disinterested witnesses that the signature to the will is in the handwriting of the person whose will it purports to be, and that the signatures of the witnesses are in the handwriting of such witnesses, or it may be proved by other sufficient evidence of the execution of such will.