‘Psychopaths need constant stimulation. The more disturbed the mind, the less gratification the perpetrator will take from each violent act,’ Ailsa contributed.

‘It’s about more than that, though,’ Ava said. ‘Serial killers refine their technique with each murder. They think up new mechanisms to try out. Most criminally active psychopaths regard the murders they commit as a work in progress. A new victim is a blank canvas. Osaki Shozo was an experiment as much as he was an expression of the murderer’s vision and capabilities. The question is, what exactly is our killer trying to show off about himself?

‘The Fenella Hawksmith scene was emotionally complex. The bindings were not only a practical method of restraint, but they also took away more than the physical ability to fight. It was like an additional punishment, a sign of what was to come. When you render a victim physically helpless, the offender is also expressing their own power.’

‘And the toaster?’ Callanach asked.

‘It’s not about the toaster, it’s about his penis,’ Ava said. ‘He’s been emasculated, reducing the victim to something less than male.’

‘Could it be retributive, maybe following a sexual assault?’ Callanach suggested.

‘Worth considering, although vigilante beatings are usually marked by a high level of out-of-control violence, typically resulting in multiple punches and kicks. There’s one punch to his face but that’s it. Overall, this is very controlled, hands-off violence. It’s structured.’

‘True, but there are cases of women waiting until their partner’s asleep or passed-out drunk before cutting off their penis. This could just be a more imaginative version of that,’ Callanach said.

‘Only the victim died, which negates the point of rendering his penis useless. Women who commit forceful penectomies usually want their victim to be aware of what’s happened to him. The payoff is in the emotional damage suffered, as much as the physical loss. It would have been obvious to whoever did this that the victim was likely to die.’

‘Penectomy?’ Callanach asked.

‘Yes. There’s actually a word for it,’ Ailsa said, shifting position to the victim’s head, peeling the tape off his mouth and sealing it in an evidence bag, then prising open his jaws. A chunk of bloodied meat fell out. ‘New evidence bag for human tissue,’ the pathologist called to a passing tech officer. ‘He bit through his tongue. Not uncommon but it’s cut clean off. The shock must have been sudden and massive. His heart would have stopped immediately.’

‘Which meant the killer chose a method that enabled him or her to enter, do what they wanted and get out fast. Why such a different scenario to Fenella Hawksmith?’ Ava mused.

‘He or she is adaptable. Mrs Hawksmith lived alone. Lock the door, don’t answer, they were never going to be disturbed. Here, there was the possibility that the wife might come home for an unforeseen reason,’ Callanach said.