Anderson reached up to stroke his chin. Collins fought the urge to reach across and slap his hand away.

‘If what you’re saying turns out to be correct, then it takes us right back to one of our original theories,’ he said. ‘The one that supposes this is the work of one of the abuse victims, taking revenge against those who wronged him, or a member of the ring itself who wants to silence the others. Here’s what’s going to happen. We’re going to dig out all of the Penvsey School case files and go to see each and every victim and each and every offender. We’re looking for anyone with the motive, the opportunity and the capability to carry out this crime. We’re looking for anyone who fits the profile that has been drawn up for us. We look at everyone and then eliminate them from our inquiries one by one. If what Dr Bernard says is true, then there are a lot more bodies to be found, and someone who was connected to that school is responsible.’

The paedophile ring that had been operating at the Penvsey School for more than two decades was uncovered not as the result of diligent police work or high-level intelligence, but rather through the stupidest of mistakes. A peripheral member of the gang had taken his home computer to a local workshop in order to repair a fault with its USB connectors. Carefully following the advice from colleagues within the ring who were far more technically savvy than he, the man had, of course, diligently erased the thousands of pornographic images and videos from the machine’s hard drive. As an added precaution, he then used special ‘shredding’ software to ensure even the most skilled technicians would not be able to find any trace of the illegal files on the machine. Rather than lose the precious pictures he had worked so hard to collect and that gave him so much pleasure, he copied them to a dozen DVDs. These he kept stored in a safe concealed beneath floorboards in the spare bedroom of the home he shared with his wife and young child. He had been astonished at just how many images there had been – it had taken many, many hours and a total of fifteen blank DVDs to store them all.

The day after he took the machine to be repaired, another member of the ring asked him to supply copies of a particular photo set that had become hugely popular among the Penvsey devotees. It was while he was searching through his copied DVDs to find the correct one that he made a horrifying discovery: there was one missing.

He had taken every possible precaution when it came to wiping information from the machine itself, only to leave one DVD full of images inside the drive of the machine.

Any hope that the repair man might not have noticed was utterly shattered just two days later when a dozen police officers arrived at his front door early one morning and arrested him for possession of child pornography.

From there the investigation grew swiftly, extending to his colleagues from work, his friends and his contacts worldwide via the internet. Before too long victims of the ring, emboldened by the fact that those responsible for the abuse were finally being brought to justice, were coming out of the woodwork from all directions.

Many of those pictured or filmed by the gang had been pupils at the school. They told members of the inquiry team how they would be summoned one at a time out of extra-curricular lessons and taken to specially set-up offices where they would be subjected to one horror after another. Threats of severe punishment and even death were made to ensure the young victims never breathed a word of what had happened to them.

At first the members of the Penvsey board of governors played down the scale of the problem, hoping all the blame could be attached to one or two errant members of staff. But it was not to be. With increasing numbers of teachers and associates being drawn into the police inquiry, it soon became clear that this was no storm in a teacup. Midway through the summer term, the school announced it would be closing its doors for ever and that the few remaining pupils – most had been extracted by their parents in the days after the scandal first broke – would have to find somewhere else to go.

The raw details of what had transpired over the years at the school made harrowing reading, but, for the first time in weeks, the members of the inquiry team felt they were finally getting somewhere. The fact that they had identified the third victim, combined with the emergence of their first credible lead about where whoever was responsible for the murders might be found, had acted like a shot of adrenalin.

In the days that followed the incident room was a hive of constant activity as phone calls were made, files studied and meetings arranged with officers from the original inquiry. While Anderson oversaw operations, tracking down the whereabouts of the victims and the remaining perpetrators was split between the two main teams: Collins and Woods, along with Hill and Porter.

A key task was to compare the profiles of both the abused and their abusers to see if any of them fitted into the patterns of behaviour identified by Dr Bernard. It did not take long for a shortlist of potential suspects to emerge and the officers set out to conduct their first interviews.

‘You seem pretty fired up by all this,’ said Woods as they made their way down to their car one afternoon on their way to see a former teacher at the school. ‘Does that mean you’ve changed your views on what we’re doing?’

‘Not at all,’ replied Collins flatly. ‘I just want to get it over with. I want to be done with it all as quickly as possible. If we don’t nail this case soon, we’re going to be working on it for the next three months at least. I can’t handle that. It’s doing my head in. I want to get a result because, once we do, we can get back to some proper police work.

‘If there’s an arrest to be made, I would rather let Hill and Porter take the lead on it. I’m sure they’ll be more than eager anyway. I don’t want to have to deal with this shit for a moment longer than is necessary.’

‘I know what you mean,’ said Woods. ‘Give me a dead drug dealer or Yardie gangster any day over this lot.’

First on their list was a 47-year-old former geography teacher named Albert Davidson who had been on the edge of the ring and served an eighteen-month suspended sentence for possession of indecent images. He had been accused by several pupils of being actively involved in the actual abuse, but insufficient evidence had meant he had managed to escape being charged.

Banned from working with children, Davidson had switched professions and now sold life insurance. Collins hated being with him. He seemed every bit as clammy and slimy as she expected a sex offender to be. He appeared completely unrepentant about anything he had done, claiming he had only started looking at the pictures out of curiosity, not out of a genuine sexual interest.

‘You had almost two thousand images on your computer. You must have been very curious indeed,’ snapped Collins.

Davidson shrugged. ‘I never counted them. I never realized there were so many.’

Short and skinny with a history of lower-back problems, Davidson was obviously not someone capable of overpowering those bigger and stronger than himself. After forty-five minutes of uncomfortable conversation, Collins was ready to dismiss him from the inquiry.

‘I feel like I need a shower after talking to him,’ she told Woods as they made their way back to the station.

‘You’re not the only one.’

‘You do realize, don’t you,’ said Collins thoughtfully, ‘that if we’re right about what this killer is up to, someone like Davidson could easily be the next victim?’

‘I try not to think about it too much,’ said Woods.

The interviews continued in a similar fashion for the whole of the next day. Some were incredibly disturbing; others just downright sad. Although the abuse was long over, it was clear that some of the victims would carry the emotional scars for the rest of their lives. Each afternoon the pair returned to the incident room to write up their statements and enter the results of their investigations into the computer database.