Robert "Bob" Harvey was the only real suspect ever attained in the investigation of the murder of Luke Doggett. There was never enough evidence to link him to the crime.
In New Orleans, LA, he died in a fiery car crash. A literal evil force lurked within the man passed on to Jeb Larold Dukes as he went on a crime spree (TXF: "Empedocles").
Later, when Doggett finally breaks the case about his murdered son, it becomes clear that Bob Harvey was a pedophile who couldn't control himself and snatched Luke Doggett right off the street. When Nicholas Regali caught Bob Harvey with Luke, Regali decided to kill the boy to keep him quiet. This probably led to a parting of ways between Harvey and Regali as well since Regali learned what Harvey was and was disgusted by him. (TXF: "Release")