US Special Agents Greg and Pete are desperate. Somewhere in America a twelve year old girl, who they have called Lucy, is being sexually abused. Her abuser has been sharing images of his abuse for six years on the dark web. They have to find her - and save her.

They were convinced that Facebook would be able to help them using its facial recognition technology, but the tech giant is unable to help. So they go back to the start, examining every single thing they can see in the room she is being abused in: the sofa, the bed and then finally they wonder what they might discover if they look at the room itself, specifically the exposed brick wall. Is there any way someone could identify these bricks? And could that person help them to find Lucy?

Season 11 of World of Secrets, The Darkest Web, is a BBC Eye investigation for the BBC World Service.

This podcast includes some upsetting scenes, discussions of child sexual abuse, and there’s some strong language.

For further information on the issues raised in the programme, contact support organisations in your own country. For a list of organisations in the UK that can provide support go to bbc.co.uk/actionline.