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‘Unpleasant, isn’t it?’ Poe said.

Cowell’s solicitor frowned. ‘I assume you think whoever put my client’s kite up that tree is also responsible for this murder?’

‘Absolutely we do.’

‘And my client absolutely denies having had anything to do with any of this,’ Lear said. ‘I would draw your attention to his earlier statement about the kite’s theft … Robert, are you OK?’

Cowell had gasped. He couldn’t tear his eyes from the top photograph on the pile. It had been exposed when Poe had picked up the one above it, the one of Rebecca’s fingers.

It was a photograph of the Secret Santa mug – the one with #BSC6 printed on the side. And for some reason it bothered Cowell far more than its gruesome predecessor.

He began trembling. His breath became ragged.

‘OK, this has gone on far enough,’ Lear said. ‘I need a consultation break.’

Chapter 39

‘My client has explained that his kite was stolen some weeks ago and this is therefore circumstantial evidence at best,’ Lear said after the interview had recommenced. ‘Now, I assume from your informal dress code that the pair of you are with some specialist unit? Why don’t you cut to the chase and show us the real evidence?’

Poe watched Cowell for a moment more.

‘As you wish,’ he said eventually. ‘Tilly, you’re up.’

Bradshaw stood and moved to the seat beside Poe. As they’d practised, she waited for him to introduce her.

‘This is Matilda Bradshaw. She’s a civilian analyst with the National Crime Agency. She’s here to explain some technical evidence.’

Bradshaw smiled and gave Cowell a small wave. ‘Hello, Robert Cowell.’

Cowell returned it. He looked confused.

‘I gather you like computers,’ Poe said. ‘Miss Bradshaw likes computers too. Please listen carefully as what she’s about to explain gets to the very heart of why we’re here.’

Bradshaw cleared her throat. ‘Thank you, Detective Sergeant Washington Poe. Robert Cowell, I’m here to explain how laser printers work.’

‘I know how laser printers wor—’

Poe had never been able to stop Bradshaw explaining science; Cowell stood no chance. She simply carried on talking. It was possible she hadn’t even heard him.

‘When you press print, your computer sends millions of bytes of information to a chip in the printer where it is converted into a two-dimensional image. The photoreceptor drum, positively charged with static electricity, begins to turn and, as it does, laser beams hit it millions of times forming the image of whatever has been sent to it.’

Bradshaw removed a diagram of a printer and pointed out each part.

‘Each spot on the drum the laser hits has its positive charge turned negative. This attracts the positively charged toner powder. Another process puts a stronger negative charge on the paper and when it is passed over the drum, the toner jumps across before being fused on at two hundred degrees Celsius. Do you understand, Robert Cowell?’

Cowell nodded. Poe did too, even though he didn’t.

‘And here’s the thing,’ Bradshaw continued. ‘Because each photoreceptor drum has its own unique, microscopic flaws, they can be treated like fingerprints. Under magnification we can match a drum to a document.’

There was no reaction. Cowell was either a very cool customer or he hadn’t realised what he’d thrown away.

Bradshaw brought up blown-up segments of the four A4 pages that had been used to wrap the Secret Santa mug. She arranged them so Cowell could see. She then pointed out the imperfections in the black swan images.

Each document had identical visible blemishes. A scratch that bisected the black swan logo in the top-right corner, what looked like a patch of dust covering two of the logos in the middle and a bubble that could be seen on a logo near the bottom.

Bradshaw had told Poe that the photoreceptor drum would have had many more blemishes but they could only be seen on the parts of paper that had logos printed on.

‘I am told that these four documents all coming from the same printer is non-interpretable evidence, Robert,’ Poe said. ‘Would you like to comment?’