 The message, recorded on an audio cassette, caused the investigation to be moved away from the West Yorkshire area, home of the real killer, Peter Sutcliffe, and thereby helped to prolong his attacks on women and hindered his potential arrest for eighteen months.More than 25 years after the event, a fragment from one of Humble's envelopes was traced to him through DNA, and in 2006 Humble was sentenced to eight years in prison for perverting the course of justice.Over the course of a year between March 1978 and the end of June 1979, Humble sent three letters claiming to be the Yorkshire Ripper.