20 C.F.R. § 219.23
(a) Preferred evidence of death. The best evidence of a person's death is—
(b) Other evidence of death. If the preferred evidence of death cannot be obtained, the individual who must furnish evidence of death will be asked to explain the reason therefor and to submit other convincing evidence, such as sworn statements of at least two persons who have personal knowledge of the death. These persons must be able to swear to the date, time, place, and cause of death.
(Approved by the Office of Management and Budget under control number 3220-0077)