In the mid 1980s in England two teenage girls were found raped and strangled. Their murderer, Colin Pitchfork, was eventually identified by his semen that he left behind through the discovery of Alek Jefferys' DNA identification.

Sources for this episode:
Wikipedia - Colin Pitchfork
Wikipedia - Newbold Verdon
Wikipedia - Leicester
The Guardian - Killer breakthrough - the day DNA evidence first nailed a murderer
BBC - Colin Pitchfork: Parole hearing for double child killer and rapist
The Daily Mail - EXCLUSIVE: The disturbing truth behind sex killer Colin Pitchfork's return to jail: Urgent calls for parole reform after predator 'walked up to' teenage women in the streetSupport the show