His smirk as he walked down the red-brick steps outside Birmingham Crown Court had made the front page of every paper the following day. Within hours, the Home Secretary had slapped a TPIM – a Terrorism Prevention and Investigation Measures notice – on Pearce, forcing him to move to Basingstoke in Hampshire, wear a tag and accept close police, and less visible MI5, monitoring.

And that was when Gabriel got involved. Pearce seemed to accept his new fate as a prisoner-and-yet-not-prisoner of the state and for five months he was followed, tagged and monitored during his trips outside the house in daylight hours. He did nothing wrong, met with no one he should not have met and lived an apparently blameless life. And then, on an otherwise unremarkable Friday, he vanished. Gabriel had been on duty that day and two hours before he slipped away she had made the call to redeploy the surveillance team to an apparently more urgent case, a supposed cell holed up next to a primary school in Camberley. The decision handed Pearce a ninety-minute window to get out of sight. He had used a borrowed car on his convoluted journey to Portsmouth, where he was pulled over by a patrol officer on the outskirts of the city. Pearce stopped, and as the officer approached, Pearce pulled a handgun and fired three times. One shot injured the officer in the leg, the second grazed the arm of a woman passing by, the third hit a schoolgirl, Isobel Harris, in the chest. Isobel was on the way home from football. She died a week later.

Isobel Harris. Gabriel had never met her, but her face was engrained in her head. In the days that followed, the press dug out a family snapshot, a thirteen-year-old girl in school uniform stood by her front door with a goofy grin and piercing brown eyes. The picture was used in every paper, on every news bulletin. She’d be fifteen by now, thought Gabriel.

Pearce fled amid the chaos and was last heard to be in Syria under his new name Abu Mujahid. Gabriel had blamed herself for Isobel’s death, and so had the tabloids, who leapt upon the tragedy as an example of incompetent policing. Gabriel’s name had emerged quickly as the figure to blame and she had endured a week of late-night calls and doorstepping. Columnists and bloggers she had never heard of called for her resignation, and there were even questions at Parliament. The story lasted ten days before running out of steam, only to resurface briefly when the inquiry cleared her of wrongdoing.

Tennant had saved her in the end, stepping in when it looked like she was about to be thrown to the wolves, persuading her to stay in SO15 and insisting to her superiors that she remain in post. She’d been unsure at first and had wondered about a move from London to another force, but she’d stayed put and was glad she had. The last year had been the busiest in counter terrorism since the IRA rampage of the late 1980s.