• 8. There is one crucial variation in the murder of Margarethe Schilling. We know the killer was interrupted in the commission of this crime, and any such variations therefore assume great significance since, under stress, we revert to what comes most naturally to us. In this instance, the deviation from pattern takes the form of the knot on the ligature binding the left ankle to the table. All other knots are simple reef knots, involving no specialist knowledge. But the odd one out is a buntline hitch, a relatively uncommon sailor’s knot.

  It is worth noting that all the cities where the murders were committed have significant access to waterways. Heidelberg and Köln are on major commercial navigable rivers – the Neckar and the Rhine. Although Leiden is no longer a commercial port, it has an extensive canal network at its heart and is close to the convergence of several major routes at Rotterdam. Given my earlier conclusion that our killer can move around Europe with ease, and given his use of a knot that most lay people would have no knowledge of, I’m prepared to go out on a limb here and suggest that it is a strong possibility that the killer is a commercial sailor, perhaps a crew member on a barge. Of course, he may simply be someone with a nautical background who is employed in another area, but I think the combination of factors gives us a strong likelihood of him being a waterman.

  Suggested action: I have no idea what records are kept of barge traffic, but I would recommend, if it is possible, that an attempt be made to ascertain whether any particular vessels were in the general area of all of these murders on the relevant dates.

Tony indulged in a moment of satisfaction. He had a good feeling about this. It was, he thought, finally getting somewhere. He didn’t know how far Petra and her Dutch friend would be able to take the case, given their limited resources. But at least he felt confident that he was pointing them in the right direction. He glanced at his watch. He had no idea when she’d be back, and he was feeling tired and grimy from his day’s travelling. He decided to head back to his own apartment, leaving a note for Petra asking her to call him when she had the chance. With luck, they could sit down later and thrash out what he’d gleaned so far. And if the gods were really smiling, she might have news for him too, if the Europol scheme had borne fruit.