McParland was named superintendent of Pinkerton's Denver office, and of the Pinkerton's western division. In April 1891, Mrs. Josephine Barnaby was murdered by poison. McParland tricked Thomas Thatcher Graves, her accused murderer, into traveling from Providence, Rhode Island, to Denver where he was arrested and convicted of the crime. McParland hired gunman Tom Horn (later executed for murder in Wyoming), who, while working for Pinkerton killed seventeen men, according to a count by Siringo. While Horn had been working for Wyoming cattlemen, "it was the cattle interests who decreed that he must die", probably to keep him from talking.