43 CFR §3837.21

1. (a)You must give the delinquent co-claimant written notice by mail using registered or certified mail, return receipt requested, or by personal service; or
2. (b)If, after diligent search, you cannot locate the delinquent co-claimant, you must publish notification in a newspaper nearest the location of the claims or sites at least once a week for 90 days.