Two heavily armed guards opened the door at the high-security Florida prison and Bernard Giles walked slowly inside.

He was 5ft 11in, balding, bespectacled, unshaven and in regulation blue jail uniform.

He seemed, on the face of it, to be a softly spoken, articulate, intelligent and rational man.

For Giles is one of America’s worst and most notorious serial killers.

Over a frenzied three-month period in the early 1970s, Giles embarked on a killing spree of unimaginable horror.

His victims were all hitchhikers whom he picked up and drove at gunpoint to remote orange groves where he sexually abused and shot them. They were Paula Hamric, a 22-year-old mother-of-two, Nancy Gerry, 18, Carolyn Bennett, 17, Sharon Wimer and Krista Melton, both just 14.

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Giles showed none of them a shred of mercy, delighting in snuffing out their innocent young lives in the most despicably violent and depraved manner. Now here he was, 45 years later.

Much older of, course, and without the long dark hair and thick moustache that he had back then. But the eyes remained the same.

As one of his former neighbours described him: ‘He had the darkest eyes I had ever seen.’

This chilling encounter with Giles will be screened on ITV on Thursday, and is the fourth of Serial Killer documentaries and by far the most unnerving.