‘It’s not much,’ Kerensa Moore agreed. ‘I’ll come to that later. There’s a very specific reason why I’m able to glean so little in this case – these cases, I should say – but I want to stay with the friendship thing for a while. Pairs of two being killed makes me wonder if Billy was the third and least powerful of a trio of friends. You’re going to ask me for supporting evidence.’ She smiled at Proust. ‘I have none. It’s no more than informed guesswork, but that’s what struck me: the rejected member of the three-that-was-a-crowd taking symbolic revenge on the other two by killing pairs of best friends.’

‘What an entertaining story,’ said Proust. ‘I have one too: Billy was deserted by his wife who ran off with her best friend after deciding she was a lesbian. Symbolic revenge. Who’s to say my theory’s any less likely than yours?’

‘Which is why I said: no evidence.’

‘You said, “informed guesswork”. Informed by what?’

‘It’s a theory based on observations I’ve made in the past, in other cases, that were later proved to be sound. One thing I am sure of: Billy is a serial killer in name only. You’re a great deal more likely to catch him if you forget everything you think you know about serial killers in your search for him.’

‘What—?’ James Wing began.

Moore raised a hand to shut him up. ‘Billy has the victim tally of a serial killer, but he lacks the correct mindset. Serial killers might have three victims, twenty-three, thirty-three or a hundred and three, but those victims are always either individuals—’

‘Everyone’s an individual,’ Dunning and Grace Woolston said in unison.

‘Apparently not,’ James Wing muttered in Simon’s ear, as Simon was thinking the exact same thing. So that was Wing’s point doubly proven.

‘The victims of serial killers, 99 per cent of the time, are individuals, killed individually,’ said Moore. ‘Serial killers rarely target groups, even groups as small as two, and in the rare cases that they do – Zodiac, for example – they kill the whole unit, be it two or three or however many, at the same time. But I can’t stress enough how exceptional that is. Normally, serial killers collect individual victims that fit a certain type.’

‘So Billy’s abnormal even within his abnormal category?’ Sam asked.

‘A freak among freaks,’ Proust summed up.

‘He’s methodologically freakish, yes,’ said Moore. ‘If he was waiting until each set of best friends was together and shooting them in pairs, I’d have a clearer basis to work from. But what Billy’s doing is nothing like anything I’ve seen before from a serial murderer. He’s choosing pairs – I mean, we have to assume he is – but he’s killing as if each victim were an individual.’

‘I suppose people with close best friends are also individuals,’ Sam Kombothekra said.