Armitage’s expression remained blank. ‘I’m all ears.’

‘I have watched and re-watched the Crimewatch reconstruction from Thursday night too many times to mention. And something has been niggling at my mind since the first time I saw it. I couldn’t place what it was at first, and then last night, it suddenly hit me. I don’t know how I didn’t notice it before.’

He nodded for Kate to continue.

‘All of the victims were found naked and in a public place, right? But his means of killing them varied each time. We also know that although they were left in public places, he didn’t kill them where they were discovered.’

Armitage glanced at his watch.

‘They were all found near their homes, with Amy discovered right outside her place in Battersea, but how did he know that was where she lived? Throughout her cover, we had her staying in a flat in Balham, working out of a local GP practice. If he found her through the online dating sites, he would have left her in Balham.’

His eyes darted back to his watch. ‘Yes, we know all of this.’

‘But can you explain it? Why did he leave her there and not in Balham?’

‘Do you really need me to spell it out? DC Spencer was an inexperienced detective, who knew little about undercover work. She got sloppy and allowed him to follow her to her real home. For all we know, he was watching her for weeks, waiting to strike.’

‘That’s my point! He knew she was a detective, and he still chose to kill her. That’s why he stopped. He knew we were close to finding him. He knew we’d worked out how he was targeting his victims. The heat was on him, and so I believe he left the country after he killed Amy.’

She paused, waiting for him to embrace what she was hinting at. ‘That’s your brilliant insight? He emigrated?’

‘Reach out to Interpol, check whether any countries have unsolved murders where the victims were left naked in a public place. That’s how to find where he is now. Let me back on the task force and you’ll see that—’

He raised his hand to cut her off. ‘Enough, Kate. You’ve wasted enough of my time already.’

‘But, Trevor, please—’

He stood and buttoned his jacket. ‘I’d prefer it if you’d address me as DCI Armitage.’

She grunted. ‘I can’t believe you’d throw your rank in my face like that. I still remember the naïve DC who came to me in tears because he’d fucked a witness in—’

‘How dare you bring that up!’

‘Don’t forget I kept you out of the firing line, and this is how you repay—’

He banged his fist against the table. ‘I agreed to see you out of professional courtesy. When the officer downstairs phoned to say you were here, the DSI told me to stay well clear. But I ignored his concerns and came to meet you anyway. Well, more fool me. Everyone around here knows how you screwed up the original investigation. You’re bad news, Kate, and the sooner you realise that, the better!’

She offered her hands in a calming gesture, and softened her tone. ‘Listen to me, okay? Just listen. I know what people around here think of me, and they’re welcome to their opinions. But you know – deep down you know – that I’m a good detective. You need me on this task force; someone who can see what’s hidden between the lines.’

He erupted into mocking laughter. ‘You think I would allow you anywhere near my investigation? It was because you couldn’t focus on what was important that this killer walked away.’

‘That’s just it – he walked away, but I don’t think he stopped. That was what struck me last night. We ruffled him because we were getting too close. But someone like that can’t just stop killing. I know he’s still out there.’

He marched to the door and yanked it open. ‘DC Jarrod, will you escort DI Matthews down to the front desk? We’re done here.’

Kate watched him leave the room and head away down the office, without looking back. She’d never felt more sure of her instincts, but she’d been unable to convince him to listen. A young woman hovered at the door, waiting for Kate to stand and exit.

Kate followed her back through the security doors to the lifts. ‘I can make my way from here,’ she said, preferring to avoid the humiliation of being escorted from the building.

The young detective nodded and headed back towards her desk.

The lift doors opened and Kate stepped in. ‘Ground floor, please,’ she said to the man who stood nearest the controls.

‘Well, this is a turn up for the books,’ he said.

Kate looked up. ‘Finn? What are you doing here?’

His face looked fresher than it had done on the television as he’d spoken about Amy and he looked genuinely pleased to see her. ‘I could ask you the same question. Here, what’s happened to you?’ he asked, nodding at the crutch.

Kate brushed off his concern. ‘It’s nothing. Why are you here?’

‘I just dropped Erin and my dad with the SIO. DCI Armitage, is it? They’re just going over details before this afternoon’s memorial service. You know Armitage, don’t you?’

She nodded.

‘He seems pretty confident they’ll finally catch the killer,’ Finn continued, as the doors opened and they exited the carriage. ‘He’s even planning to station undercover officers at the graveyard for tomorrow’s memorial service, in case anyone suspicious decides to show up. Listen, I need to kill some time until they’re ready for me to drive them home. Do you fancy going for a coffee somewhere?’

If anyone had an inside track on Armitage’s investigation it would be Finn – at last Kate’s luck was changing.