On March 27, 1991 — three months after he was taken off the investigation of a 1990 double murder — former city police Det. Vaughn Maher authored a memo discrediting an informant who had provided evidence that pointed towards one particular suspect. Maher’s memo helped shift the police department’s attention away from a man named Michael Cardwell and towards the two men who would ultimately be convicted of the crime: Scott Lewis and Stefon Moran…