Gripping the brick, I started trying to lever it out. It wasn’t easy: even without the cement around it, there was very little space to get my fingers along the top, but I kept at it, gently rocking it from side to side and pulling towards me every time it did.

Eventually, it dropped out, clinking against the granite.

I grabbed my phone, switched on the light and shone it into the dark of the cavity wall. At first I couldn’t see anything, but then I spotted it: a small black unit, almost entirely hidden from view on the extreme left of the hole.

‘What can you see?’ Healy asked.

‘Something’s been put inside here.’

Reaching in, I grabbed hold of the unit and brought it out. It was roughly the size of a mobile phone and, because it was clearly switched on, it was warm to the touch. As I held it in my hand, showing it to Healy, it buzzed gently, alive; from one end of it, wires snaked off into the cavity wall, still connected to somewhere else.

‘What the hell is that?’ Healy asked.

‘It’s a hard drive.’

He frowned. ‘A hard drive? For doing what?’

I looked at the wires coming out of it.

They were audio leads.

‘I think someone’s been listening to us.’

Late-Night Call

1985

Los Angeles | Tuesday 23 July

As soon as Jo got back to the station, she started ticking boxes. All detectives had to fill out a form that outlined the basic components and elements of the murder they were investigating, which acted as a memo to the Sheriff and, in theory at least, would be read by every officer in the department so that patterns could be spotted and acted upon more quickly.

It took her an hour, and then she spent another hour going over teletypes, the overviews of crimes that had taken place within Los Angeles County, looking to see if she could find any links between the man in the tub and other murders that had occurred inside the nearly 5,000 square miles that the department covered. There was nothing.

Pretty soon after, the phone on her desk burst into life.

‘Jo, it’s Dan Chen.’

‘Danny. What you got for me?’

‘Looks like the tub was filled with muriatic acid. I thought it might be as it’s easy to get hold of. You can pick it up in any hardware store with no checks at all.’

‘That’s a cleaning compound, right?’

‘Yeah. Folks use it for brightening concrete, masonry, that sort of thing. It’s in pool solutions and drain cleaners as well. I’ll know for sure once we’ve done a proper autopsy but I’d say he was in there two days max. Muriatic acid is a lower-grade hydrochloric acid, less pure, but it still does the job. If he’d been a week in that stuff, we’d have been scooping him out with a spoon.’