O'Brien provides a brief history of the uses of handwriting and printed document analysis, followed by his best-chosen case reference: the New York Zodiac Killer . O'Brien mentions the notes written by Zodiac Killer, but he does not detail the full role the letters played in the case. “For years, the Zodiac taunted police with strange codes, phone calls, insulting and cryptic messages. . . . The [one-digit] key was handwritten on a sheet of white paper and was accompanied by a short typewritten note on a 3x5 index card expressing the hope that "the enclosed key will be of use to you in connection with the 'author of the encrypted letter'. It was signed “concerned citizen” (crimelibrary.com). Providing full details supports Holmes' approach in using documents (written and typed) as well as the file containing the