I was packing my kit bag for the gym. Scout was off for a sleepover with her friend Mia, and down on the street the meat market’s night was just beginning. After the day I had spent at Marble Arch, I knew that sleep would be a struggle for me if I did not exhaust myself at Fred’s.

Then Edie called with what felt like our first breakthrough.

‘The good news is we’ve got prints,’ she said. ‘All our forensics are back for Mahmud Irani and Hector Welles and the same print is on both of the victims’ clothes.’ I could hear the excitement in her voice. ‘It’s a glove print, Max, but really sharp. A thumb. A left thumbprint on both of the dead men.’

Most criminals believe that gloves hide fingerprints. But it is not true, especially with more modern gloves made of latex or something similar. The thinner the glove, the more likely the telltale ridges, whorls, arches and loops are to be left behind.

‘And what’s the bad news?’ I said.

‘None of it rings any bells on IDENT1.’

IDENT1 is the country’s major database for storing fingerprints and contains the fingerprints of knocking on for ten million people. That only leaves about fifty million people who are not on there – the part of the population who have never come into contact with the police.

‘And both of our potential suspects are on IDENT1,’ I said. ‘Because both Paul Warboys and Barry Wilder have criminal records.’

‘Wilder for his youthful indiscretions at the football, and Warboys because crime was what he did for a living,’ Edie said.

‘Are we sure it’s not them?’

Fingerprint analysis is not the exact science that it is always cracked up to be in the movies. Fingerprint officers have been known to get it wrong. Until 2001, a sixteen-point standard existed for fingerprint matches – meaning there were sixteen identical points required on a latent print to legally match it to a suspect. The system was scrapped because it didn’t work.

‘It’s not even close, Max.’

‘But we still don’t have the kill site, so we don’t have prints on surfaces, do we?’ I argued. ‘Paul Warboys or Barry Wilder could have glove prints, fingerprints, footprints and DNA all over the kill site. This one print found doesn’t mean either Barry Wilder and Paul Warboys – or both of them – weren’t there. It doesn’t mean they had nothing to do with it.’

‘It makes it a lot less likely though, doesn’t it?’

I had to give her that. ‘Yes.’

Jackson came into the loft, soaked in sweat from his evening run. Stan got off the sofa and padded across to greet him. The pair of them stared at me talking on the phone.

‘You know what this means, don’t you?’ Edie said.

‘There’s a very strong possibility that these guys don’t have criminal records.’