In chapter 54, Magwitch and Compeyson begin a fight and go overboard in the water where Magwitch either kills Compeyson, or Compeyson is overcome by wounds/water/injuries. No one knows exactly what happened besides Magwitch (who confesses little) and Compeyson, because it was difficult to see. What we do know in chapter 55 is that Compeyson is dead and Magwitch is wounded. In the beginning of 55, since Compeyson is dead, investigators have to call another witness to identify Magwitch because his accuser, Compeyson, was dead. Jaggers, of course, refuses to comment on the identity.