So they’d tossed suggestions back and forth until finally they came up with something that had the feel of possibility to it. Marijke, who was flavour of the month with Maartens, had managed to convince her boss that she should take part in the pursuit. She had hired a twenty-nine-foot leisure cruiser with a couple of berths, a tiny galley and a pungent chemical toilet. The idea was to maintain visual contact with the Wilhelmina Rosen as she made her way up the Rhine towards Holland. If Mann appeared to be targeting another victim en route, the German police would do what was necessary. But if they made it over the Dutch border without incident, Tony would attempt to confront Mann and extract evidence from him, with Marijke’s team providing back-up. It had taken all Marijke’s powers of persuasion, but she’d eventually convinced Maartens to go along with the stratagem. The temptation of being the man who would succeed where the Germans had failed had proved too much in the end. Petra had supplied them with a state-of-the-art surveillance kit: a tiny radio microphone embedded in a pen whose signal could be picked up on a remote unit by Marijke. As soon as Tony had elicited enough evidence, Marijke and her colleagues would play the cavalry and come riding to the rescue.

It was a strategy fraught with risk, but Tony had been as resolute as Marijke that Mann’s killing spree had to be brought to an end. ‘With the last killing, the level of violence leapt dramatically. Now he’s overtly sexualizing his murders, he’s going to want to enjoy them more often. There’s no reason why he should confine himself to Germany and Holland either. When it gets too hot for him in one place, he can simply cross a border and begin again. We can’t hang back and wait till he finally makes a mistake that provides something harder than circumstantial evidence. I won’t sit on my hands while a whole community is staked out like a sacrificial lamb,’ he’d said to her as they’d boarded their boat.