What are some of the clues those are leading that Mr. Douglas and Mr. Barker are guilty

Introduced at the beginning of the story as John Douglas, this man appears to be a faceless victim, simply the object of Holmes’ investigation. Towards the end of Part I, however, Douglas is revealed to be very much alive, and he commences to share his history, which comprises nearly the entirety of Part II. During this “story within a story," we learn that Douglas once bore the name “Jack McMurdo," and that he was part of a notorious gang in the United States, which was not unlike the Mafia.