‘These codes and symbols may seem childish, but you must be familiar with them – you may see them on tattoos, graffiti, T-shirts, football shirts, email addresses, letters. I’ve got a full list here – I warn you, there are dozens. These people are obsessed.’

She handed a wad of papers around the room.

Gabriel was staring at the screen. Bose had closed down the video file, and its title was on display.

‘DI Bose, is that the original title of the video file?’ she asked.

‘Yes, ma’am. We got it from the internet streaming company. Forensics have looked at the file, it was uploaded at an internet cafe in Central London, the video was probably shot in the last few days.’

‘Enlarge it, please.’ Gabriel scribbled down the filename. ‘420chiA_C. Anything in that?’

Skeffington interjected from the back.

‘Looks pretty random, ma’am,’ he said.

‘I disagree,’ said Gabriel, scribbling away. ‘420, according to DI Bose’s list, is Adolf Hitler’s birthday. Chi could be Chilton. But what’s A_C?’

‘Maybe Fletcher was a fan of Alice Cooper?’ said Skeffington.

Gabriel ignored him. ‘Something combat? Or it could be one underscore three if you use the alphabet code in reverse?’ she said.

‘But they don’t tend to do that, ma’am,’ said Skeffington.

She shot him a withering glance. ‘They don’t tend to kill people on roller coasters either, DI Skeffington, and yet here we all are.’

#

Back at her desk, Gabriel looked through Bose’s briefing notes. The lack of intel on the NRF and the extreme far right was frustrating. There must have been a similar problem in the late 1990s when the threat from the Northern Irish paramilitaries was ebbing in the wake of the Good Friday agreement, just as the seeds of a new type of Islamist terror were being sown around the world. Then there must have been some urgent adjustments, some re-juggling of old thinking and a desperate shortage of intel about these new jihadists.

It was hard to imagine a time when the IRA was the biggest threat on the mainland. Gabriel wondered if her dad had been involved in counter terrorism. The skills of surveillance riders were easily transferrable from serious crime to organised crime to counter terrorism. She’d never thought to ask him when he was alive. Another wasted opportunity.

She had been asked in her interviews for SO15 whether her father’s career in the Met had influenced her own job choices. It was an obvious question, but one she had never actively considered until then. She had just known since she was ten that she wanted to join the police and the fact that her dad was in the service was simply part of the furniture of her life, like the green wardrobe in her bedroom.

Gabriel had enjoyed a speedy rise through the ranks. She’d graduated from Nottingham University and been one of the few women to get on the police’s High Potential Development Scheme, a fast-track career path for graduates. She worked for four years as a beat bobby in Surrey and then quickly moved up the ranks, getting an early promotion to DI aged just thirty-three. She married Ray a couple of years later and moved to the Met to work in the Directorate of Professional Standards. She had been happy enough there and had risen to rank of detective superintendent, but it was not until the summer of 2017 – the summer of Westminster Bridge, London Bridge and Manchester – that she had felt her first real calling and had requested a transfer to counter terrorism. A vacancy came up six months later.