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Specialist online investigators had spent months on their mission to save a 12-year-old girl trapped with a sexual predator.

Pictures and videos of her abuse were being shared to approximately 400,000 people on the dark web - a portion of the internet originally created by the US Department of Defense to ensure its spies operated in secrecy.

These days, it is more commonly the online playground of many of the world's worst paedophiles.

Lucy, the child Homeland Security officers were desperately trying to save, had featured on that sinister portion of the internet since she was just seven.

Officers, however, could not track her down because posts on the dark web are not linked to an IP address leaving predators able to operate without a direct trace.

It meant that the team, headed by specialist investigator Greg Squire, had to dig for clues elsewhere.

Greg's work is at the centre of a new Storyville documentary The Darkest Web, which airs tonight on BBC Four at 10pm.