Relevancy of statements: signed responses to summons may be admissible if the declarant is unavailable or the court permits. A signed statement made in response to a summons during inquiry or proceedings is relevant in prosecution to prove its factual content when the maker is dead, cannot be found, is incapable of giving evidence, is kept away by the adverse party, or whose presence cannot be obtained without unreasonable delay or expense; it is also admissible if the maker is examined as a witness and the court considers admission in the interest of justice.