Wynne had been sceptical when she first spoke to Tony Munro, the Australian detective – she was glad he had called, but how likely was it that a series of murders nearly a decade ago on the other side of the world were linked to what was happening in South Warrington right now?

Not very, was the answer, and in her experience long shots rarely paid off.

It was, though, the only thing she had to go on. There were no other leads. No CCTV, no DNA that they could trace, no witnesses. That was the thing with detective work that most people misunderstood – they thought cases were solved by high tech forensics or images from satellites or sudden, Sherlockian flashes of inspiration, but the truth was much more prosaic. Most cases were solved because someone saw something and told the authorities, or because the criminal made a stupid mistake and left their wallet at the scene. Most often of all was the humble confession; under questioning, few people could hold out.

And in this case she had none of those things. Not even a suspect.

So the Perth murders were all she had to go on, and although she wasn’t hopeful at first, the more she thought about them, the more she became convinced they were linked.

A savage blow from a hammer to the head. Late at night, in a deserted location. No pattern, and nothing at the scene that spoke of a ritual element to the killings.

Just a random, efficient murder in the dead of night.

Untraceable, inexplicable, and, without some stroke of luck, unsolvable.

She read the notes she had made. Six killings, terminating abruptly in mid 2013. The detectives on the case had speculated as to why they had stopped: perhaps the murderer moved away or was hurt or went to prison for some other offence. It was impossible to know.

She paused and tried to put her thoughts in order. She had no more evidence, no new suspects, but she had learned – potentially – that there was a link to the earlier murders. It wasn’t much, but it was something.

She scrolled through the files on the screen. There were hundreds. Each detective had been assigned a group of them to review; if they found nothing, they would swap their group with another detective so that two sets of eyes saw each file. It was tedious, but it was the only way.

She had her files to review, too. She clicked on one, and began to read.

There was a knock on the door. It opened and DS Alexander stepped inside. She was holding a mug of tea and a piece of paper.

‘Here,’ she said, and put the tea on the table. ‘I just had one.’

‘Thank you,’ Wynne said. ‘I appreciate it.’

‘That’s not the only reason I’m here,’ DS Alexander said. ‘There’s this.’

She handed the paper to Wynne.