A key rule of crime detection was never to jump to conclusions, but it was difficult not to immediately think of the red-masked, SIG-toting bandits who had killed Andy Northwood. Whatever debris remained of these slugs, when they managed to prise it out of the walls, that would be all the connection they needed. But Lucy couldn’t do that herself.

She went back down the corridor to the top of the stairs, feeling in her pocket for her mobile. But before she could place a call, another long, low groan of timber reverberated downward.

She glanced at the exposed laths above.

Old, half-forgotten buildings like this spoke to themselves all the time, but it was obvious that this wasn’t what she’d heard. And now she heard it again. And again, and again – until it became a pattern, the regular tremors of which shuddered through the very walls.

Lucy’s scalp prickled as she realised she was listening to what sounded like a rocking chair.

She hadn’t passed an upper stair on the way to the office where the blood and bullet holes were, so it stood to reason that any access to the rooms above lay in the other direction, beyond that veil of darkness left of the stairway. Common sense, not to mention procedure, screamed at her that she should get out of here, retreat outdoors and call for support. But she resisted it, that old-school conscience kicking in; she wasn’t just some concerned citizen, she was a copper, and she had a crime scene to preserve. Not only that, offenders might still be on the premises, and if she simply exited stage left, what was to stop them slipping out another way?

Face taut with tension, nerves like piano strings, she intruded into that darkness at the other end of the corridor. When she activated her phone light, it revealed a floor, walls and ceiling that were black with rot.

When an open doorway appeared on her left, literally from nowhere, she half jumped.

There was no door in its rectangular frame, just an empty blackness, but Lucy shone her light into it, revealing another uncarpeted stair leading upward.

As soon as she put her foot on the bottom tread, the creaking overhead ceased.

That was almost enough to send her into retreat, but damn it, she wouldn’t – not when she’d come this far. Steeling herself with the knowledge that backing off now was no solution whatsoever, she climbed to the top, where a circular skylight was covered by several decades of coagulated autumn leaves – but where the light leaking up from downstairs was just sufficient to show another single door standing ajar by about an inch.