To the left was a fir hedge littered with faded cans. Ahead, where the path swept to the left, a greyish animal scurried out from the hedge, sauntered across the path, turned to stare and then slowly squeezed itself through the fence and out of sight.

‘Tell me that was a squirrel, Campbell,’ said Gabriel.

‘It was a squirrel, ma’am,’ lied Campbell.

‘Bollocks it was. That was a rat, wasn’t it?’

‘Yes, ma’am. But more spooked by us than we are of it.’

‘Don’t believe a word of it,’ said Gabriel. ‘That little shit wasn’t scared of anyone. Did you see the look he gave you? That was the look of someone eyeing up dinner.’

They continued down the path. She led the way, taking easy big strides over the puddles while he scurried along in her wake.

Some people grow into their features as they age. Some try to cling to their youth. Gabriel had simply stopped caring quite so much. She was aware that her face was angular, her nose on the beaky side and her complexion pasty. Her hair, cut in a sharp bob, had been intended to soften her features, but it merely accentuated them, and she was stuck with it for now. At almost five foot nine inches, her height had advantages when challenging suspects and male colleagues, but she considered herself too gangly, more like a freshly sprouted adolescent than a woman in the grip of middle age. She wore a simple charcoal suit, which looked like Marks & Spencer but was actually something more expensive from a designer store in Chelsea.

She and Campbell made a contrasting pair. With his thickset features, he looked every inch – or rather every five foot seven inches – of reliability. People looked at Campbell and the word that sprang to their minds was ‘safe’. That was a useful attribute in a sergeant, thought Gabriel. It meant people opened up to him.

From the waft of rancid cooking fat, it was clear they were approaching the park kitchens. They had a couple of minutes before they would be at the crime scene, so she ran through the facts with Campbell. She planned to talk to the victims’ families later and needed to be on top of the details.

‘Gareth Fletcher, white, twenty-one, lives alone,’ she said. ‘One previous for minor assault. No known political affiliations. And – surprise, surprise – a bit of a loner. His colleagues said, and I quote, “He kept himself to himself”.’

Campbell grinned. It was a running joke in SO15 that every serial killer and terrorist was described by neighbours and work colleagues as a loner who kept themselves to themselves.

Gabriel continued. ‘In addition to this startling insight, according to his nearest and dearest – or his boss at least – Fletcher had never expressed a particular urge to murder anyone. Until today when he took it upon himself to stab a dozen fee-paying passengers, mostly Asian, before fleeing via the staff car park.’