43 C.F.R. § 3837.21
How do I notify the delinquent co-claimant that I want to acquire his or her interests?
(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
(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.