Winter nights are hard in prison. Hours dilate and darkness is absolute.

On Tuesday 9 January the sun set over London at 16.11.

Children were walking home from schools; parents had welcomed in the final hour of the working day. For all inmates at Wormwood Scrubs, association time had been cancelled after a fatal stabbing at the weekend. The cells were bolted at sundown, and they didn’t open again for sixteen hours.

No disturbance was reported in the night. No sounds of distress echoed between bars. Even the rats, it was later said, made themselves scarce.

At dawn, they found the bodies.

Thirteen men had died in their cells.

Some had choked. Others had bled. It wasn’t long before rumours spread through the wings. Whispers of suicide pacts. The governor called it a tragic coincidence, a bizarre succession of unfortunate accidents. The coroner reported death by misadventure.

Nobody at the prison dared to call it what it was until the tabloids did it for them.

The Prison Service was facing a locked-room mystery spread out across three impenetrable wings. Thirteen bodies in a single winter night.

What happened should’ve been impossible.

Somehow, it was murder.