To be fair to the scientists, it probably didn’t help that their Bradford Park base was openly referred to as ‘Geek Corner’ and ‘Virginville’ by certain officers such as Rowlands.

Jessica and Cole were shown through to a waiting area where they were told someone would come to see them. On occasion, officers would be permitted into sterile zones and autopsy rooms but there was no real need at this point. Jessica thought the room they were shown to was actually quite attractive, brightly decorated with a royal-blue carpet. The chairs they were offered were low to the floor but the material was bright red and comfortable. The person who led them through said they would bring some tea without asking if they even wanted one.

It was a far cry from the waiting rooms in their station. Back there, you would be offered a metal and plastic hard-backed seat like the ones you found in a school and the only refreshment on offer would be dodgy-tasting tea from a machine.

Jessica was just getting comfy in the chair, fiddling with her phone, when a man pushed open a glass door into the room. He was wearing dark trousers and shoes with a laboratory coat over a shirt. He had shoulder-length black hair and looked as if he hadn’t shaved for a few days. His stubble was dark and Jessica would have guessed he was around her age. As he came through the doorway, he tripped seemingly over nothing and Jessica giggled.

The poor guy looked embarrassed as he walked across the room towards them.

‘Detectives Cole and Daniel?’ The two officers stood up to shake hands with the man whose face had gone slightly red. ‘I’m Adam Compton. I’m one of the team who did the blood work on the body of Craig Millar.’

The three of them sat around a glass table. Jessica started speaking but they were interrupted by the receptionist returning with mugs of tea, putting them on the table. After she had left, Jessica began. ‘Mr Compton . . .’

‘Adam.’

‘Sorry, Adam. We just wanted to clarify a few things with you about the testing procedures. Obviously results like this aren’t what we would usually get.’

Adam nodded along in agreement. ‘How can I help you then?’

‘The obvious question is: could the results be wrong?’

Jessica didn’t want to sound as if she were accusing him of making a mistake but, given the fact Donald McKenna was very much behind bars, it was a question that had to be asked. She softened her tone as she spoke.

Adam sounded nervous. ‘That was what we thought at first. My boss wasn’t, erm, happy. He thought I had made an error cross-checking things with the database.’

‘Okay,’ Jessica said. ‘We may as well go back to the start for completeness’ sake. Can you talk us through the whole database procedure . . . and, er, feel free to talk to us as if we’re complete idiots.’

She knew most of it but hearing it from someone who knew for sure would clarify things. Either way, she didn’t want a stream of technobabble.

Adam’s accent definitely wasn’t local. Even face to face instead of over the phone, she couldn’t place it. ‘What happens is that every time you arrest someone, you take those mouth swabs, don’t you?’ Jessica nodded. ‘Those swabs give us a sort of pattern that is unique to the individual it’s taken from. They are stored by various companies but that pattern is kept in a database that all sorts of agencies have access to.’

‘That’s the National DNA Database, yes?’

‘Yes, the NDNAD.’

‘That sounds like some kind of STD.’ Jessica laughed quietly at her own joke but neither Adam nor Cole joined in and she quickly stifled her giggles into a fake cough.

Adam continued. ‘Say there’s a crime scene where you find hairs or blood or something like that, the people who work at the scene try to get as clean a sample as possible . . .’

‘How do you mean, “clean”?’

‘For instance, if you touched it with your fingers, you could transfer your own profile onto what you were picking up, which would contaminate it.’

‘Right.’

‘So anyway, assuming the sample is clean we would analyse it to get whatever pattern we could from it. That would then be matched against the victim to see if it belonged to them. If it doesn’t and there’s a chance it could belong to whoever committed the crime, we instead . . .’

‘. . . check the pattern against the database’ Jessica said, finishing Adam’s sentence.

The scientist smiled at her. ‘Exactly, yes.’

Jessica thought he seemed a bit awkward, perhaps nervous. He was quite fidgety, almost as if his body constantly wanted to be somewhere else. He started to continue speaking but the receptionist returned, telling him he had a phone call. He apologised and said he would be right back.

Cole picked up his mug of tea and took a big gulp. ‘I think you’ve got another admirer.’

Jessica laughed under her breath. ‘Stop it . . .’

‘Seriously. The poor guy can barely get his words out.’

‘Maybe it’s you he fancies? Perhaps he swings that way?’

Cole was still laughing as Adam returned, sitting back down. Jessica could see what her superior was talking about now. The scientist would glance up at her but not want to catch her eye, looking at the table while he spoke.

Jessica picked up where they left off. ‘I’m guessing that from the blood you found under Craig Millar’s fingernails, the profile of that matched back to Donald McKenna?’

‘Yes.’

‘So what happened then?’

‘Then I got shouted at for getting things wrong.’

‘Really?’

Adam looked a little embarrassed again. ‘Sort of. Look, mistakes can sometimes be made in labs. If samples haven’t been kept correctly or someone hasn’t followed the procedures or so on, like I said, they can be corrupted.’

‘But they weren’t in this case?’

‘We don’t think so, no. If there’s something that doesn’t seem right, we go all the way back to the original sample and re-test that, rather than rely on the pattern stored in the database. It doesn’t happen very often and usually takes days.’

‘How come you got it so quickly then?’

‘It only takes time because these samples are stored all over the country and it’s only the actual database itself which can be accessed anywhere. Because Mr McKenna was someone local, it turned out we were storing his original swabs.’

‘And it all matches?’

‘Yes.’

Cole spoke. ‘Is there any chance someone else could have the exact same, er, “pattern” that Donald McKenna does?’

‘Theoretically, yes but not really. It’s something like a one-in-a-billion chance of someone else having the same DNA profile. I guess there are six or seven billion people in the world so someone could but even that’s very unlikely.’

Jessica hadn’t checked for anything like birth certificates but had seen on their records that Donald McKenna had no known relatives. She felt she had to ask the question anyway. ‘What about a brother or something like that?’

‘You share half the same genes with your siblings or parents. What would happen then is we would see a partial match. Say for instance someone like Mr McKenna’s brother had done something, we would get that partial match to Mr McKenna himself and know it was someone related to him by the first degree. If it was an uncle, we might get a second-degree match or third for a cousin or something.’

‘It sounds simple.’

‘It’s not.’

‘Okay, so you’re saying it has to be his blood then and no one else’s?’

‘There is one other possibility. If you had an identical twin, you would share the same profile. It would have to be identical though, like you came from the same egg. Non-identical twins would show as first-degree matches like a regular brother or sister.’

Jessica looked at Cole, who spoke directly to her. ‘Does he . . . ?’

‘Nothing in the files I saw,’ Jessica said. ‘We’ll have to check the birth records properly when we get back but, if he did have a twin, I can’t believe it wouldn’t be on our system.’