He became aware of Maria Klee standing beside him.

‘Why do they do it?’ Fabel spoke as much to himself as to her. His eyes ranged over the images.

‘Why do they do what?’

‘Why do they comply? The first victim seems to have met with the killer by arrangement. Her car was found parked and locked up in an autobahn rest station and there was no evidence of a struggle or violent abduction. This second victim … it’s like she invited her killer in. Or that he had a key. There’s no evidence of forced entry, or of a struggle at or near the threshold. I suppose, to a certain extent, you can understand a prostitute being, well, welcoming. But Ursula Kastner was an intelligent and safety-conscious young woman. Why did they both comply with a complete stranger?’

‘If he was a stranger,’ said Maria.

‘If he follows the typical serial-killer profile then, as you know, he will not pick victims that have prior knowledge of him …’ Susanne Eckhardt now joined Fabel and Maria.

‘So why did Kastner go with him and “Monique” let him in?’ Fabel repeated his question. Maria shrugged.

‘Maybe there was something about him that invited trust …’ Susanne paused, as if weighing up her own words. ‘Do you remember the case of Albert DeSalvo?’

Maria and Fabel looked at each other blankly.

‘Albert DeSalvo. The Boston Strangler. He murdered a dozen women in Boston in the early sixties …’

‘What about him?’ Fabel’s confusion was genuine.

‘The Boston police asked exactly the same question you’re asking: “Why did the victims let him into their apartments?”’

‘So why did they?’

‘DeSalvo was a plumber by trade. He would call at the door and say the apartment-block supervisor had asked him to call. If the victim was suspicious or protested, DeSalvo would simply say, “Okay,” and walk away as if it was no big deal to him. Because the victims didn’t want to cause trouble with their landlords, because DeSalvo obviously had the authentic tools of his trade with him and because he didn’t push the issue, they would call him back and open the door.’

‘So what are you saying?’ asked Maria. ‘That we should be looking for a plumber?’

Susanne sighed impatiently. ‘No, not necessarily. But it is possible that he is masquerading as something similar. Something that invites trust, even though he is a stranger to the victim.’

Maria tapped her pen against her teeth. ‘We know that this guy is, by his own admission, anonymous in appearance. Maybe he gets off on dressing as someone in authority before killing …’

‘Now, that, Herr Fabel,’ Susanne Eckhardt revealed her perfect teeth in a broad smile, ‘is much better amateur psychology than yours.’

Fabel’s eyes ranged over the images on the wall. ‘Let’s say he does embellish his ritual by dressing as an authority figure. What would epitomise authority to the victims, as well as gaining their implicit trust?’

Maria Klee stared at Fabel for a moment. When she spoke, it was almost a whisper. ‘Oh, shit.’