John Melvin Preston (July 8, 1945 – June 2, 2008)[1][2] was a con artist, American dog handler, and former state trooper from Pennsylvania[3] who testified for the prosecution in criminal cases across the U.S. in the early 1980s.
[2] Preston claimed that his dog could smell human traces years or months after a suspect walked over the ground,[7] on heavily trafficked streets,[2] or both.
[9] Preston's testimony was repudiated by the Kings County District Attorney in New York and the Arizona Supreme Court, who called him a "charlatan".
I believe that Preston was regularly retained to confirm the state's preconceived notions about a case.
[3][12] The 2004 review did not flag the case of Bill Dillon, who was later freed on DNA evidence contradicting Preston's testimony.
[14] The Innocence Project of Florida believes that as many as 60 people may have been convicted based partially or solely upon Preston's testimony.