He looked over at her before he answered, sounding almost amused. ‘Nothing.’

‘Then why target them?’

‘Well,’ he appeared to lift one of Mike’s eyelids, ‘it’s really a matter of pragmatism. I exploited their weaknesses to gain an advantage. Survival of the fittest, you see?’

‘I get the analogy. But that doesn’t explain why you murdered them.’

‘No.’ White laughed quietly. ‘I suppose not.’

He stood and went to the corner of the room, as Hawkins strained her neck to keep him in sight. She changed tactic, eager to sustain dialogue. ‘You said you exploited their weaknesses; not we. What about the rest of your group, aren’t they involved?’

‘The society knows nothing about this. It’s … a personal project.’

‘To what end?’ She waited, but he ignored her, kneeling beside a black shape on the floor that looked like some kind of holdall.

‘And why attack us?’ she pressed, her voice faltering. ‘You must have known we were coming, or you wouldn’t have hidden outside in the trees. You could have escaped.’

White glanced up. ‘Yes, but you were already on to me. That’s why you’re here, isn’t it, because I failed to hide my intrigue when you mentioned this place earlier today? I genuinely didn’t think you’d piece it together this fast, but when the cleaner rang my mobile a few hours later, while I was here setting up, to say you’d returned to Cavendish Square looking for me, I realized my mistake.’

‘Your mistake was using John Mardell as a distraction, and expecting us not to track him down.’

He seemed to nod thoughtfully. ‘You’re right, that was short-sighted of me. But in my defence, I was meant to be long gone before now.’

Hawkins winced as she coughed. ‘I don’t understand.’

White didn’t answer her question. ‘When I knew you might come here, I hoped you’d bring young Constable Whelan, but I suppose beggars can’t be choosers.’ He turned back to the holdall and reached inside, removing something she couldn’t identify until he shook it, producing the unmistakable sound of a spray can agitator.

He moved over to a light patch of brickwork and began spraying black lines on to the wall. When he stepped away, painful concentration allowed Hawkins to make out what he had drawn on the pitted, uneven surface:

The pentagram, a symbol they had seen several times during the course of this investigation. Except here, just like the ones scrawled on the outside of the tower, this version was upside down. She remembered Chris’s words from a few days earlier:

It’s a sign of satanic worship.

Her gaze drifted after White, who was repeating his actions on the adjacent wall, although this time she didn’t recognize the mark: