Next to Thorne, Kitson scribbled and mumbled, ‘Like a jigsaw puzzle.’

‘Except we don’t have a box with the picture on,’ Thorne said.

‘Right, let’s crack on.’ Brigstocke took one last look at the screen. ‘But if anyone fancies doing some major arse-licking and spending every minute of their spare time trying to figure that out for us, I’ll be extremely grateful.’

‘Better than a bloody sudoku,’ Karim said.

Brigstocke smiled. ‘Not that anyone’s going to have any spare time, you understand.’

As exaggerated groans broke out either side of him, Thorne stared, unblinking, at the picture. The sequence of numbers and letters.

‘As yet, we’re none the wiser…’

He imagined the killer working with nail scissors, his face creased in concentration. Pictured him later sweating and bloodstained, carefully laying each piece into a victim’s palm and folding the dead fingers around it.

‘There are obviously pieces missing…’

Thorne stared at the gaps.

Half an hour later, when the team had dispersed, Thorne and Kitson wandered across to Brigstocke’s office for a less formal briefing. For the DCI, daily sessions like this were a chance to catch up with senior members of his team and talk about ways to take the inquiry forward. To air grievances, or talk through ideas someone might be too embarrassed to suggest in a larger meeting. A year or two earlier, the cigarettes would have come out; before that, back in the days of Cortinas and fitting up Irishmen, the secret stash of Scotch or vodka.

When Thorne and Kitson arrived, the door to Brigstocke’s office was open. He was on the phone, but as soon as he saw them, he beckoned them inside and motioned for Kitson to shut the door.

Thorne saw the expression on Brigstocke’s face and did not bother to sit down. He had a good idea what was being talked about when the DCI said, ‘You’re sure, because this one sounds different.’ He knew, by the time Brigstocke was talking about pieces of plastic and press blackouts.

Thorne exchanged a look with Kitson, and waited.

Brigstocke hung up and let out a heartfelt groan on a long, tired breath.

‘Another piece of the jigsaw?’ Thorne asked.

The blood had still not returned to Russell Brigstocke’s face. ‘Two of them,’ he said.

The bodies of Gregory and Alexandra Macken, aged twenty and eighteen, had been discovered just after 9.30 a.m. by the landlord of their rented flat in Holloway – an Iranian named Dariush – who had come round to fix a leaking radiator. They were informally identified by the elderly woman in the flat downstairs, who claimed to have heard them coming home on Saturday, two nights earlier, but had not seen them since.

‘They came back at different times, and there were definitely two male voices earlier on.’ She was very insistent about that, while also making it clear that she didn’t like to stick her nose into other people’s business. Later, when she was tearful, she said, ‘Nicer than your average students.’ She made sure the uniformed officer wrote that down. ‘Didn’t make a racket and always said hello. Even fed my cat when I went to stay with my sister.’

Both victims were found dead in the larger of the flat’s two bedrooms. Gregory was discovered naked on the bed, while his sister, who was wearing pyjamas and a dressing-gown, was found on the floor. Both had slivers of dark plastic in their hands, and head wounds that were clearly visible through blood-spattered plastic bags.

Within an hour, the CSI team had set to work. A uniformed WPC from the local station did her best to comfort Mr Dariush in preparation for taking his statement, while a family liaison officer was sent to talk to the next of kin and inform them that they would be required to identify the bodies formally the following day.

If they felt up to it…

‘I never understand why anyone would choose to do that,’ Thorne said. ‘I mean, most of us have to do it at one time or another, but why would you sign up for a job where all you do is deal with other people’s misery? Where you have to… absorb it?’

‘Because you’re empathetic?’

‘Because you’re what?’

‘You give a toss.’

‘All the time, though?’ Thorne shook his head and swallowed a mouthful of coffee. ‘I’d rather face somebody with a gun.’

‘You should think about retraining,’ Hendricks said. ‘The dead ones are no bother at all.’

It was almost six o’clock. After more than seven hours at the crime scene, Thorne and Hendricks had left the flat as evening fell and walked the few hundred yards to a coffee bar on the Hornsey Road, to kill time while they waited for the bodies to be brought out.

‘How much longer has the brother been dead?’

They had taken a corner table without needing to discuss it, both well used to staying as far away from other customers as possible whenever they found themselves in a bar or restaurant and there was shop to be talked.

‘Ten, twelve hours, maybe,’ Hendricks said. ‘The sister’s been dead around a day, so he would have been killed more like thirty-six hours ago.’

‘So, Saturday night and Sunday morning?’

Hendricks nodded and took a slurp of tea. ‘Good film that. When Albert Finney was still gorgeous.’

‘You think he’s gay?’ Thorne asked.

‘Albert Finney?’

Thorne ignored his friend and waited. He had been thinking about what the downstairs neighbour had said and was working out a timeline. The girl had not been killed because she had interrupted someone in the act of murdering her brother.

The killer had waited for her.

‘Look, I’ll be able to tell you if anything sexual went on tomorrow,’ Hendricks said. ‘Who did what to who, and for how long. Macken was definitely gay, if that helps.’

‘Gaydar work on corpses, does it?’

‘He had Armistead Maupin and Edmund White on his bookshelves and Rufus Wainwright on his CD player.’

Thorne had heard of Rufus Wainwright. ‘I’ll take your word for it.’

‘The killer might well be gay too,’ Hendricks said. ‘But if you ask me, he’s whatever he needs to be. Does whatever he has to do to get through the front door.’

‘Then whatever he has to do once he’s inside.’ Thorne finished his coffee, spoke as much to himself as Hendricks: ‘He… adapts.’

‘I’m not sure we’ll ever know exactly what happened,’ Hendricks said. ‘How he got the girl in there, whether he was hiding. But this time he brought two plastic bags with him.’

‘Two bags, but just one blunt instrument,’ Thorne said. They had found a heavy glass bowl by the side of the bed. There was hardened candle-wax in the bottom of it and what looked like brain-matter and dried blood caked across the underside. ‘He plans things carefully and he thinks on his feet.’

Hendricks nodded. ‘He’s good at this.’

A waitress came over and asked if they would like more drinks. Hendricks said that he was fine, but Thorne ordered another coffee, happy enough to sit there for a while.

‘What does he do for twelve hours?’ he asked.

‘What does who do?’

‘Our man, after he’s killed the boy.’

‘Maybe he sleeps,’ Hendricks said. ‘Reads a book. Has a wank.’ He shrugged. ‘I know what it looks like inside these nutters’ heads, but don’t ask me what goes on in there.’

Thorne leaned back in his chair. ‘Has a wank?’

Hendricks grinned. ‘A lot of these are sexual, right?’

Some were, certainly, but Thorne had already decided that these killings were not sexually motivated, and not only because of a lack of evidence. A violent death was never treated as something ordinary, but when it was about sex or revenge or money, there could at least be some level of understanding. When it was about none of these things was when it got scary.

And Thorne was starting to feel afraid.

They both started at the sudden banging on the window, turned and saw a drunk who had tottered past once already, pressing his big red face against the glass. Thorne looked away but Hendricks began to smile and waved at the man. The waitress, who was hovering at a nearby table, apologised and moved towards the door, but the drunk, having blown one final kiss at his new best friend, was already lurching away along the pavement.