The way a knot is tied and the type of knot used might have been a priceless lead, I thought, remembering Pat Whitestone’s words when we had found Mahmud Irani. But Elsa shook her head.

‘Sorry,’ she said. ‘For that I would need to see the rope or the wire or whatever it was. The neck markings are not enough to go on.’

‘We still don’t have a kill site, Elsa,’ I said. ‘How long had they been dead when we found them?’

‘They’re all different. As you know, rigor mortis sets in after approximately two hours and then the body gets progressively stiffer. And then we follow the twelve-twelve-twelve equation. Twelve hours to get stiff. Twelve hours to remain stiff. And twelve hours for the body to lose that stiffness as it begins to decay. Rigor mortis had just begun to set in on Mr Irani, suggesting he had been dead for around twelve hours when he was discovered. The second victim, Mr Welles, had advanced rigor mortis. He had been dead for approximately twenty-four hours. And the body of the latest victim, young Mr Donovan, was losing the stiffness of rigor mortis. And you can see that his skin has a greenish hue around his head, shoulders and abdomen. There’s also a degree of bloating because of gases accumulating in the cavities. His internal organs had longer to break down.’

‘Because the bodies are becoming harder for us to find,’ I said.

‘And there’s lividity on the back of Mr Irani,’ Elsa said, nodding at Edie. ‘Would you be so kind, DC Wren?’

Together the two women turned the cadaver of Mahmud Irani onto his front. The skin around his shoulder blades, back, buttocks and calf muscles was pale and surrounded by ugly purple blotches that looked like bruises. Dr Joe looked at me.

‘That’s the lividity,’ I said. ‘Those marks that look like bruises. Think of lividity as stagnant blood. When you die, your heart stops beating and your blood stops moving. Gravity does the rest. The blood settles. But you don’t get it where the body touches the ground. Lividity can help us determine how long someone has been dead and if the body has been moved.’

‘The technical name is livor mortis,’ Elsa said.

‘From the Latin,’ Dr Joe said. ‘Livor meaning bluish and mortis meaning of death.’

Elsa and Edie heaved the dead man onto his back.

‘The lividity on Mr Irani strongly suggests that he lay undisturbed for most of the time between death and discovery,’ Elsa said.

‘What does that mean?’ asked Dr Joe.

‘It means they didn’t move him very far,’ I said. ‘So the kill site has to be within – what? – one hour’s distance of Marble Arch and Vauxhall Bridge.’

‘That narrows it down to all of Greater London,’ Edie said.