What is more significant is that he has allowed this breach in his self-control to occur. I believe this may have come about in part because he was disturbed mid-murder in Bremen. This must have unsettled him to a considerable degree, making him much more nervous when approaching Dr Calvet. I believe he will have shocked himself with his actions in Köln. To maintain his earlier level of denial about the erotic nature of what he was doing, he probably convinced himself he had some kind of altruistic mission. But now he has descended to rape, it will be harder for him to maintain the integrity of that delusion.

What does this mean for detection and prevention?

I believe he will try to kill again very soon, perhaps within a matter of days. He has to restore his vision of himself as some sort of avenging angel or righter of wrongs, to erase this momentary lapse into the behaviour of what he may well see as an ‘ordinary’ criminal.

If I am right that he is somehow connected to the waterways, then his options may be limited to quite a small geographical area. I believe the time has come when his potential targets should be informed of the risks. I would urge that this be done in a low-key manner to avoid alerting the killer. Officers should identify university departments with an experimental psychology specialism and make personal visits to the campuses. They should stress the importance of maintaining confidentiality if they are to have the best chance of capturing the killer, and they should invite co-operation. Lecturers who have been contacted about interviews for a new on-line magazine should be identified. This could allow a sting to be set up. If this is done quickly, it may prevent a fifth killing.

Tony read over what he’d written, then sent it to Marijke and Petra, with a copy to Carol. From what Marijke had told him, it looked as if the cases were already getting bogged down in red tape, with everything being routed through a secure area in the Europol computing centre at Den Haag. He hoped that, between them, they could inject a sense of urgency into the investigation. Otherwise, they were all going to end up with more blood on their hands.

Tadeusz walked Carol to the door of the apartment block. ‘Thanks,’ she said. ‘It’s been an interesting evening.’

He took her hand and bowed deeply over it, planting a kiss on the back of her hand. ‘Thank you for coming. I’ll call you, yes?’

Relieved that he wasn’t angling to come up for coffee, Carol nodded. ‘I’ll look forward to it. Good night.’