The Vatican Robbery by Joseph E. Riley

Two honest brothers, forced into crime by criminal threat planned the perfect crime to steal valuable Vatican relic in cased in a jeweled receptacle. Their plan succeeds but during escape one brother is killed in an automobile accident. The surviving brother is the only one who know details of the crime and where the relic is hidden, but he suffers from amnesia as a result of the same accident. Famous insurance investigator is stymied. The story details what happens to the surviving brother and how the investigator gets involved. Surprise ending.