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‘I think we’re in a good position to take it to the CPS, ma’am. We have enough to charge Robert Cowell with murder. We’ll charge Rhona with abduction but keep investigating her.’

The chief constable said, ‘Poe, you were in the room with him – what are the obvious sticking points?’

Poe considered the question for a few moments.

‘I don’t know why he killed three people when the instruction was just to “kill a stranger” and I don’t know why he used different methods to amputate the fingers. If the yellow dot tracking didn’t link him directly to all three murders I’d have said that the Cowells weren’t the only people playing the Black Swan Challenge.’

‘Could Rhona Cowell be behind all this?’

‘I thought that, ma’am, but if she is, why expose herself?’ Poe said. ‘She’s almost certainly going to prison for the abduction.’

The chief constable nodded. ‘Anything else?’ she said.

‘I’d like the printer,’ Poe replied. ‘Cowell’s defence is going to be that he’s being framed – it’s his only option – and by saying his kite was stolen he’s already sowing the seeds for that. If we find the printer we negate a large part of his strategy.’

‘Where are we on tracing this administrator, Tilly?’ Nightingale said. ‘Tell me you have some good news?’

‘No, Detective Superintendent Nightingale,’ she replied. ‘He’s used his own websites to host the chatrooms in which he spoke to Robert Cowell and presumably anyone else playing the Black Swan Challenge. This is the URL for one of them.’

She brought up a fresh screen.

dhwehi234o8757o4632obf.onion

‘I’m assuming that’s a dark web address?’

Bradshaw nodded. ‘They all are.’

‘You said he used more than one?’

‘It was like a game of whack-a-mole. He would use a site for a limited period of time then shut it down. As soon as he did, another would immediately go live.’

‘What’s that “onion” thing at the end of the URL?’

‘That’s the TOR extension. It’s how he hosts his websites.’

‘TOR?’

‘The Onion Router. It’s free software that enables anonymous communication. TOR directs internet traffic through thousands of relays, which makes it difficult to trace the user’s location. He hosted all his Black Swan Challenge chatrooms on the TOR Network.’

Poe didn’t pretend to understand what Bradshaw was saying but he silently cursed the dark and deep web – the 96 per cent of the web that wasn’t discoverable with standard search engines such as Google. Living at Herdwick Croft, he had every sympathy for anyone wanting to protect their privacy, but as far as he could tell, the dark web was just a marketplace for weapons, drugs, hitmen and indecent images of children. And now serial killers by proxy had started using it. As if their job wasn’t hard enough already.

‘You said difficult to trace, not impossible?’ Nightingale said.

‘I can normally trace the user’s IP address using TOR entry and exit points but in this case all we ended up with was this.’

Bradshaw reached down and retrieved a paper evidence bag. One of the brown ones with a clear window. It contained a flat piece of electrical equipment.

‘This is a single-board computer,’ she said. ‘It is basically a computer where all the components it needs to be functional have been built into a single circuit board. It has microprocessors, storage and memory. Single-board computers are normally embedded into larger devices like ATM machines, cash registers and medical equipment, but anyone can buy one. They are not expensive. The model in the evidence bag can be bought online for under thirty pounds.’

Bradshaw changed the image on the monitor again. The exterior of a scruffy-looking guesthouse appeared on the screen. It looked like the type that specialised in the transient and hard-to-house population. It would be skimming money from their housing benefit and would have police officers there every other week.

‘This is on the outskirts of Carlisle. I traced this single-board computer to a bedroom on the top floor. It had been plugged in, connected to their free wi-fi network then hidden under the floorboards. It was recovered not an hour earlier. The guesthouse owner had no idea it was there.’

Nightingale had already been briefed on this part but she asked the question for the benefit of those who hadn’t.

‘And this is as far as you can trace?’

‘It is. He managed it remotely for a short period of time then moved on to the next one he’d hidden. After he’d finished with this he remotely factory reset it, so we are struggling to recover any digital evidence. I have traced five computers in total. I understand detectives and CSI teams are recovering the remaining four as we speak.’