‘How come none of this showed up on the PNC search?’ asked Anderson.

‘The cases were in Humberside,’ Collins said simply.

‘Oh, for fuck’s sake.’

No further explanation was needed. Every officer in the country knew that, up until the mid-2000s, Humberside Police had a policy of destroying all computerized intelligence files. Senior staff at the force had misinterpreted the Data Protection Act and decided they would be in breach of regulations if they kept information about those who were arrested or questioned but never actually charged. No other force in the country made the same error.

Humberside’s actions only came to light when it emerged that Ian Huntley, the Soham murderer, was able to get a job in a school despite numerous complaints that he had raped or sexually assaulted under-aged girls. Computerized background checks run by Cambridgeshire Police failed to throw any light on his past because the information had been erased.

‘What about the wife? Why didn’t she say anything before?’

‘I think she was ashamed. She saw he had an interest in much younger women, but, so far as she knew, he never did anything to pursue it, at least where under-aged girls were concerned. She hoped it was just a phase he was going through and that, over time, he would fall in love with her as she was, not just yearn for the early days of their relationship when she was little more than a schoolgirl.

‘She hinted at what was going on when DI Hill interviewed her, saying her husband was looking to replace her with someone younger, but he never pursued it.’

Anderson hit the speaker button on the front of his phone, then dialled a number. After two rings the gruff voice of DI Hill filled the room. There was a rushing sound in the background and Collins guessed that he was in his car.

‘Hello?’

‘Hill, you’re a fucking idiot.’

‘You what?’

‘Just get yourself back here a.s.a.p.’

Hill’s voice spluttered as he tried to work out why on earth his boss was delivering such a bollocking. He had barely begun to string a sentence together when Anderson hit the button again and cut him off, turning his attention back to the people in his office.

‘Good job, Collins. I guess DCS Higgins was right. Let me introduce you to someone. This is Dr Jacques Bernard. He’s a criminal psychologist with King’s College. I brought him in to help us work up some kind of profile of the person we’re looking for. Dr Bernard, this is Detective Inspector Stacey Collins, one of the lead officers on the murder team.’

Dr Bernard stood up, moved towards Collins and held out his hand. ‘It’s a pleasure to meet you,’ he said. His voice was beautifully mellow. Their palms touched. His hands were large and powerful yet neatly manicured. When Collins went to pull hers away, he held on just a fraction too long.

‘Nice to meet you too,’ she said softly.

Anderson handed the report that Collins had given him to the psychologist. ‘Do you think this could be relevant?’

Dr Bernard frowned in concentration. ‘It’s very hard to say. At the moment, so far as I’m aware, there is nothing to link the other two victims to this kind of activity. It could be nothing more than a coincidence, but I think it is undoubtedly a lead worth pursuing.

‘There have been cases where killers have targeted, say, prostitutes in a belief that they are cleaning up the streets, ridding the world of its filth. Going after sex offenders could be part of a similar mission.’

Anderson nodded. ‘But, as you say, it could be nothing more than a coincidence. I remember talking to a friend about the Soham inquiry. You know the night those girls went missing, a convicted paedophile from Devon who had previously abducted a couple of girls around the same age happened to be in town. Turned out his granny lived there. He had nothing to do with the crime, just a pure coincidence. These things happen. Collins, let’s keep this material out of the briefing for now – I don’t want anyone’s thinking blinkered as a result – but I want you and Woods to follow this up in the morning. Top priority.’

It took half an hour or so before DI Hill and the remaining members of the inquiry team who were out and about returned to the office and arranged themselves in a semicircle to await the promised presentation.

Dr Jacques Bernard moved to the front of the room until he was standing directly underneath the board showing the names of the victims. He took in the faces of those around him, his eyes seeming to linger for a moment when they met those of Stacey Collins, who was sitting on a desk towards the back. Dr Bernard cleared his throat and began to speak.

‘I can tell from the expectant expressions on your faces that you assume I’m about to perform some kind of minor miracle, that I’m going to tell you exactly what sort of person you’re looking for, where they live, how many kids they have, how long they’ve been married and what they do for a living.

‘But sadly this isn’t some slick American television show and I can’t give you anything of the sort. What I can do is tell you everything that I know about serial killers. And you may be surprised to find that much of what you think you already know is likely to be wrong.’

Collins shifted in her seat and glanced over at Woods, who rolled his eyes up at the ceiling. She didn’t like profilers at the best of times and felt that this one, though more pleasant on the eyes than most, was already starting to get on her nerves. It was as if she was back at school with some know-it-all teacher lording it up at the front of the class. She hadn’t liked school much either.

‘Statistically, the typical serial killer is a white male from a lower-to-middle-class background, usually in his twenties or thirties,’ Dr Bernard continued. ‘He is not a lunatic. He is far more likely to be a charming, impeccably dressed, polite individual than an out-and-out oddball. Like the best predators, serial killers stalk their prey by gaining their trust. Don’t go out there looking for someone who stands out from the crowd: the person you’re after will be able to blend in perfectly.

‘He will be absolutely fascinated with the police and with authority in general. He probably spends a lot of time reading about police procedures, taking courses or watching detective shows. He may have attempted to become a police officer himself but probably got rejected and ended up working as a security guard or serving in the army. He may even have found a job that brings him into regular contact with the authorities. One thing that is particular to this case: the offender is going to be extremely physically strong, capable of lifting a full-grown man.

‘It’s likely that he also spends a lot of time reading books about psychology and sociology. He will use that knowledge to manipulate situations to help generate sympathy in those around him. This, of course, is the perfect way to get people to lower their defences.’

Collins folded her arms and shifted back on the desk in an effort to get comfortable. This might have been new to some of the others on the team but for her it was old hat.

Dr Bernard continued. ‘In terms of actual background, you’re looking for someone who may have been physically or emotionally abused by their parents. Some serial killers claim that exposure to violent images or events was the catalyst that sent them down the path of destruction, but the fact is that there are many children and adults who have witnessed such events and gone on to become perfectly normal, productive members of society.

‘Although the popular image of a serial killer involves someone who takes a trophy from his victim or tortures and mutilates them, the reality is that this happens only in a tiny proportion of cases. Most of the time killers hit victims over the head, rape them or strangle them, and leave them wherever they drop. It usually happens very quickly. You’re looking for someone who has the ability to put a lot of time and effort into what they do.