Sachs had a thought. ‘And if it took awhile, that meant she was in pain for a long time.’

‘Well, yes, you can see the pain reaction but I have a feeling that was later. She couldn’t have been conscious while he was writing his message. Even if she wasn’t trying to get away, the involuntary movement would’ve ruined his handiwork. No, he subdued her somehow. Any trauma to the head?’

She examined the woman’s scalp carefully and looked under her blouse, front and back. ‘No. And I don’t see any signs of Taser barbs. No stun gun welts … Ah but, Rhyme, see that?’ She pointed out a tiny red dot on her neck.

‘Injection site?’

‘I think so. I’m guessing sedative, not poison. There’s no sign of any swelling or other irritation that toxin would cause.’

‘The blood work will tell us.’

Sachs took pictures of the wound and then bent down and swabbed the area carefully, lifting trace. Then the rest of her body too and the ground around her. It was likely that a perp this diligent would have worn gloves – it certainly appeared that way. Yet valuable evidence from even a gloved-and-gowned perp could still easily be transferred to the victim or crime scene.

Edmond Locard, the French criminalist who lived a century before, formulated the Exchange Principle: that every time a crime occurs there is a transfer of evidence between criminal and scene, or criminal and victim. That evidence (which he referred to as ‘dust’) might be very, very difficult to detect and collect but it exists, for the diligent and innovative forensic scientist.

‘There’s something odd, Rhyme.’

‘Odd?’ A splinter of disdain for the artless word. ‘Go ahead, Sachs.’

‘I’m using only one of the first responders’ spotlights – the other’s pointed up the tunnel. But there’re two shadows on the ground.’ She looked up and walked in a slow circle to get a clear view. ‘Ah, there’s another light near the ceiling, between those two pipes. It looks like a flashlight.’

‘Not left by the first responders?’

‘What cop or medic is going to give up his Maglite?’

The big black tubed flashlights that all cops and firemen carried around were invaluable – great sources of illumination and they doubled as bone-breaking weapons in a clutch.

But she noted it wasn’t one of those expensive models. This was cheap, plastic.

‘It’s taped to the pipe. Duct tape. Why would he leave a light here, Rhyme?’

‘That explains it.’

‘What?’ she asked.

‘How the store manager found the body. The flashlight. Our perp wanted to make sure we found the message from our sponsor.’

The words seemed a little flippant to Sachs but she’d always suspected that much of Rhyme’s gruff façade and sardonic delivery were defense mechanisms. Still, she wondered if he raised the barricade of protection higher than he needed to.