How should we handle 1099s for retirees we can’t track down?

Question: Three retirees receive monthly pension checks. Two have stopped cashing them and the post office keeps returning the other retiree’s check as undeliverable. We’ve searched the obituaries and looked in our old HR files for their next of kin, but we’ve come up empty. We have to provide Forms 1099-R to them by the end of this month. What do we do?