‘Yes, but have you any idea how much commercial traffic there is on the waterways of Holland and Germany? There must be thousands of craft on the rivers, and our man could be on any of them. I don’t even know if there’s any record kept of their movements. I spoke to Marijke briefly this evening, and she seemed to think that boats have to register when they go through locks or tie up at wharves, but that still doesn’t narrow it down much, and ploughing through all that material could take months. We haven’t got months, Carol.’

‘And even if they warn potential victims, it might not be any help in catching him,’ Carol said.

‘That’s right. It’s possible he might just go to ground temporarily and resurface with a new strategy for cornering his victims.’

‘If he’s on-line, might there be any mileage in checking with the internet booksellers to see who’s bought a wide range of psychology textbooks?’ Carol asked.

Tony shrugged. ‘If he lives on a boat, it would be easier for him to buy his books in a shop rather than have them sent to an address he might not get to for a few weeks.’

‘I suppose,’ she said, trying not to sound too dejected. ‘What about the Stasi angle?’

‘Petra has arranged for me to talk to a historian tomorrow. But again, I think we’re going to be doing needle-in-a-haystack stuff.’

‘I’m interested in what he thinks he’s doing here,’ Carol mused. ‘If you’re right, and he thinks his life has been screwed up because somebody close to him was a victim of mental torture, what’s his goal here? Is it vengeance, pure and simple? Or is he trying to send a wider message?’

‘Well, it depends on whether we’re talking conscious or subconscious motives here,’ Tony said. I’d say that subconsciously he’s trying to get his own back. But that’s too personal, too petty for him to acknowledge as his primary motive. I think he sees himself as cleaning the Augean stables of psychology. ‘He’s sending a message out – if you mess with people’s heads directly, you deserve to die.’

Carol frowned and fiddled with her coffee cup. ‘I know this is going to sound off the wall, but do you think he sees what he’s doing as a kind of cure? A form of ultimate therapy? Now you won’t indulge your horrible destructive habit any more?’

This was what Tony loved about working with Carol. Her mind sloped off laterally and came up with ideas that he would either never have or would have dismissed as too improbable for consideration. She’d done it before, and she’d been right when he’d been wrong. ‘You know, that’s not a bad idea,’ he said slowly. ‘But where are you going with this?’