Terrifying letters depict a man on the edge consumed with sick sexual fantasies involving the abduction, rape, and murder of innocent young women.

Despite a DNA profile, fingerprints, and even the discovery of a forest torture chamber, gardaí get no closer to identifying the author of the tomes.

And today, former garda handwriting expert John Sweetman tells Nicola a terrifying story from his investigation files which form an episode of his new podcast Lines of Enquiry.

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