The hunt for the Yorkshire Ripper was disastrously derailed in its latter stages when letters and tapes arrived at the operation's headquarters addressed directly to George Oldfield - the man in charge of the investigation, and the public face of the police's efforts.
So convinced was Oldfield of the importance and genuineness of these documents that he ordered their use as a direct means of eliminating suspects. In effect, officers were told to discount any suspect if he didn't have a Sunderland accent or his handwriting didn't match those of the letters. As a direct result of this, Sutcliffe was effectively eliminated from enquiries, despite having been the subject of questioning over the £5 note found in the handbag of Jean Jordan in Manchester and his repeated appearance in the heavily-surveilled red light districts of several cities in the north. Some blame the hoaxer for enabling Sutcliffe to remain at large for almost two further years.
This belief also diverted thousands of precious man hours into fruitless door-to-door enquiries in the Castletown area of Sunderland and the creation of a deluge of "helpful" phone calls from people who believed they recognised the voice - all of which had to be investigated thoroughly.
Thousands of billboards and newspaper adverts were printed with samples of the handwriting which desperate police hoped would lead to the killer's door, while the tape was played endlessly on television and radio.
In fact, the tapes and letters were a crude hoax - although the perpetrator remained unidentified until 2006. The story of the capture of John Humble is told in this documentary.