John Preston

John Melvin Preston (July 8, 1945 – June 2, 2008) was an American con artist, dog handler, and state trooper from Pennsylvania who testified for the prosecution in criminal cases across the U.S. in the early 1980s. or 2 ) could perform feats of forensic detection far beyond the abilities of other investigative dogs.

As an expert, Preston was paid $300 per day; in his previous job as a Pennsylvania state trooper, he was paid $20,000 per year. He claimed that his dog could smell underwater, and, in a case against a man who was eventually freed on DNA evidence, Preston claimed that his dog could track a human scent even after hurricanes. Preston was never charged with any crime.

Effect and fallout Preston helped convict at least two men who were eventually freed by DNA evidence. The 2004 review did not flag the case of Bill Dillon, who was later freed on DNA evidence contradicting Preston's testimony.

Preston testified as an expert witness at dozens of criminal trials in Florida. The Innocence Project of Florida believes that as many as 60 people may have been convicted based partially or solely upon Preston's testimony. Florida Today found 15 cases in which Preston testified.

Preston's testimony resulted in over 100 criminal convictions.