‘How?’ asked Bolt.

‘The lens has been spray-painted black. The camera itself is about ten feet up an apple tree in the garden, facing the back door. My guess is the suspect shinned up the tree and did it.’

‘Can we take a look?’

Black nodded. ‘Sure. Follow me.’

As they walked down the driveway and round the side of the house, Bolt noticed a pool of vomit congealing on the front doorstep.

‘That came out of the first officer on the scene,’ Black told them matter-of-factly. ‘Apparently, the two upstairs were his first murder victims. And, as I’m sure you can imagine, it’s not very pretty up there.’

Bolt flashed back to the last Disciple murder scene. The blood and the horror. ‘Poor sod,’ he said. ‘I don’t think you ever get used to that kind of thing.’ But he noticed that Black didn’t seem unduly concerned by what he’d witnessed. Maybe some cops did get used to it after all, and he was the exception rather than the rule.

The rear garden was several hundred feet long and about half as wide, and was enclosed by the same high brick wall that lined the front of the property. Mature bushes and foliage ran along the borders, while beech trees from the encroaching woods loomed up beyond the wall. Apart from the occasional birdsong and the faint hum of rush-hour traffic from the nearby A33, the woods were silent.

‘The camera’s up on this tree here.’

Bolt followed the angle of Black’s finger to a point about halfway up an apple tree that stood a couple of yards beyond the patio at the back of the house. There was still plenty of foliage on it, and even a few mouldy-looking apples. ‘I can’t see a thing,’ he said.

Mo shook his head. ‘Neither can I.’

The two of them approached the tree, and only when he was at the bottom looking up could Bolt make out the black box partly concealed by a branch. It would have been easy enough for the suspect to have got up there and spray-painted it, but only if he knew where he was looking. He turned to Black. ‘How the hell did he know it was there?’

Black shrugged. ‘He’s your killer, DCS Bolt. Not ours. I thought he was meant to spend a lot of time casing the homes of his victims before he went in. If he knew what he was looking for, he would have found it eventually.’

It was a fair point, and once again it made Bolt realize how carefully their killer picked his targets and planned his attacks. ‘Can you take us through what happened here?’ he asked.

‘We haven’t interviewed the woman who survived the attack, but her name’s Amanda Rowan and she’s the wife of the male victim, George Rowan. From what we can piece together, she came home and disturbed the killer, who was in the process, or had just finished, murdering George Rowan and a woman we’ve tentatively identified as 29-year-old Ivana Hanzha. Miss Hanzha is listed on the payroll of the investment bank that Mr Rowan works for.’

‘So we can assume she was Mr Rowan’s lover.’

‘It looks that way. Anyway, we think Mrs Rowan’s arrival wasn’t expected, either by the victims or the killer. There was some sort of confrontation and then the killer chased her out of the house.’

‘You said there was CCTV at the front,’ said Mo. ‘Did that capture anything, or was that tampered with too?’

‘No it wasn’t, and yes it did,’ answered Black. ‘We examined the footage a couple of hours ago and it clearly shows Amanda Rowan entering the property at nine fifty-three p.m. last night, and then exiting again through the front door at nine fifty-six, being chased by a man dressed entirely in black and wearing a black ski mask. He grabs her and slashes her on the arm with a knife, but she manages to wriggle out of his grip and then disappears from shot. After that, it seems she ran to the nearest neighbour – an old lady called Vera Naseby, who lives about a hundred metres away. We haven’t interviewed her yet, but she was the one who called 999 and alerted us to what was going on.’

‘It looks like Mrs Rowan had an incredibly lucky escape,’ said Bolt.

Black nodded. ‘Even luckier than you think, because the killer continued to chase her. When Mrs Naseby answered the door, he was right behind her. He followed her into the house and up the stairs. She had to jump out of Mrs Naseby’s bedroom window to get away from him.’

Bolt and Mo exchanged puzzled glances. The Disciple might have been deranged but he’d always shown himself to be an efficient operator. ‘Maybe he’s more hot-headed than we thought,’ said Bolt slowly.

‘Maybe he just doesn’t like leaving witnesses,’ said Black.

‘You said in the footage that the suspect was wearing a ski mask. So why risk chasing a witness through a neighbour’s house, especially if she hasn’t seen your face? It just seems unduly reckless.’

Nobody had an answer to that one, but Bolt was hoping the reason was because The Disciple was becoming caught up in his own hype, and now believed he was untouchable. It was a reasonable enough explanation. Sociopathic killers experience a real sense of power from ending the lives of their victims. If they get too much of it, and think they’re more powerful than they really are, they end up doing stupid things.

‘Come on,’ said Black. ‘Let me show you inside.’

They stopped at the open back door – a standard half-glass PVC design – and checked the lock. It was undamaged, but this didn’t tell them anything. They already knew that The Disciple had housebreaking expertise, since he’d got into the other three properties through locked doors.

‘So we reckon he must have been watching the place a while, seen that the couple were inside. Then, when the coast was clear, he came in this way.’ Black led them into a huge, spacious, open-plan kitchen, where a couple of white-suited SOCO were dusting for prints. Even downstairs, the smell hit Bolt immediately – a sour, pungent stink of turned meat – and he tensed. He would have given a month’s wages then not to see the bodies, but knew he was going to have to. He needed to be sure that this was the work of the man he was after.

They followed a line of red markers on the floor through the kitchen and into an even bigger lounge with three separate sofas and a huge plasma TV taking up the majority of one wall. Ahead of them was the front door, next to which was a pair of high-heeled court shoes, one of which was on its side. Amanda Rowan would have kicked them off as she came in the door, presumably having no idea what was about to happen to her. A staircase ran up to the next floor and, as Bolt followed Black up it, he imagined The Disciple creeping up here, knife in hand, listening for sounds of life. It took an amazing amount of self-confidence to enter a home like this, not knowing your way around it, and hunt down your prey, knowing that you were always one against two.

The smell grew stronger.

‘I’ve got Vicks if you need it,’ said Black, taking out a handkerchief and putting it to his nose as he reached the top of the stairs. He motioned to an open door at the end of the landing, behind which a couple of white-uniformed SOCO were working.

Bolt shook his head and moved past Black, wanting to get this over and done with, Mo following him. The two SOCO looked up as Bolt walked into the bedroom, then turned back to what they were doing as he inspected the scene grimly.

It was a bloody mess like all the others, the bodies still lying where they’d fallen for the last time. The male victim – George Rowan – was tied to a chair at the foot of a large iron-framed double bed, facing towards it. He was naked, and there was a deep knife wound in his lower thigh just above the knee, which had bled profusely. There were other wounds to his abdomen, face and groin too, where he’d been deliberately cut by his attacker. But it was the thigh wound that Bolt was most interested in, because this was the typical first strike of The Disciple. It was how he liked to operate – sneaking in and disabling the male half of the couple by stabbing him in the thigh, before restraining them both. Rowan’s head was slumped forward, his grey-black hair obscuring his face, but Bolt could clearly see that his throat had been cut.