‘I can also tell you they were all taken down minutes after death,’ Elsa said. ‘If they had been left hanging, the furrow of the neck wounds would be much deeper.’

We stared in silence at the bodies. I felt myself shudder.

It’s just the cold, I told myself. Just the bitter cold in here.

‘And there’s something else,’ Elsa said. ‘A professional hangman works on a system of variable drops. It was the method used by this country’s most famous executioner – Albert Pierrepoint. Saddam Hussein’s goons used the same method. How heavy the body is, how far it needs to fall to separate the second and third vertebrae in the neck and cause instant death. It’s called the hangman’s fracture. If the drop is too long, the victim is decapitated. These people – the Hanging Club – clearly don’t bother with variable drops. Despite the charade of hanging, they essentially strangle their victims.’

She gave Edie Wren a severe look.

‘And I watch YouTube, too,’ Elsa said. ‘But what’s interesting is that the most recent victim appears to have died in less than half the time of the first two. Examining the damage to the carotid arteries and the spinal column, I estimate that Mr Irani took thirteen minutes to die, Mr Welles took ten and Mr Donovan took just five.’

‘What does that mean?’ asked Dr Joe.

‘It means they’re getting good at it,’ I said.

We were back in the changing room, taking off our scrubs, when Dr Joe began talking about signatures.

‘The three murders are more than ritualistic killings,’ he said, pulling off his hairnet. ‘Among the unknown subs there’s a clear hierarchical structure at work.’

‘Yes, it’s always the same one who asks the question,’ I said. ‘Do you know why you’ve been brought to this place of execution? He’s the leader.’

‘Everyone has their role to play and yet they are capable of acting in the interests of the group,’ Dr Joe said.

Edie pulled off her hairnet and shook her hair out. ‘When Hector Welles tried to do a runner, they all jumped on him at once. Or at least, three of them did. And then they went back to their roles. And they all have a strict role to play, don’t they? Somebody to speak. Somebody to film. Somebody to watch. And somebody to be executioner.’

‘Judge, jury, witness and hangman,’ I said. ‘But what are the signatures, Dr Joe?’

‘A strong facility for organisation. An ability to be totally ruthless. A strict hierarchy that has room for individual endeavour before the hierarchy reasserts itself.’

‘You really think they’re cops?’ Edie said.