Finn and Kate opted for an Italian deli just down the road from New Scotland Yard because there appeared to be enough room to sit without being overheard by the people on the next table. She didn’t like to guess how many of the other customers were somehow connected to the police.

He insisted on buying the drinks, and also ordered some biscotti to share. Once seated, Kate asked him how much Amy had told him about the undercover operation.

‘Nothing really,’ Finn responded. ‘But we all suspected she was on the investigation for that serial killer, because every time the story was mentioned on the news, she’d insist we quieten down, and turn up the volume. She didn’t like to talk about the job, I think, because she didn’t want to worry her mum.’

Kate nodded. She’d also preferred to keep quiet about work when she first joined the force. But secrecy wasn’t an issue now her mother’s dementia had worsened. Now she couldn’t even be relied on to remember Kate’s name.

‘I’ll never forget the voicemail from my dad on that Saturday morning, saying something terrible had happened and that he wanted me to phone him immediately. I thought maybe Erin had been taken ill, but…’

‘I’m sorry to make you relive it.’

He sipped his coffee. ‘You don’t have to apologise. I want you to find the bastard who did this.’ He paused, taking a moment to look at her directly. ‘What were you really doing upstairs this morning?’

‘I asked DCI Armitage to add me to his task force—’

‘That’s great!’

She lowered her eyes. ‘He said no.’

Finn’s cheeks reddened. ‘Oh.’

Kate shrugged apologetically. ‘I’m sure he’s got the situation in hand. They wouldn’t have appointed him if they weren’t confident he could draw a line under all this. But know this, I won’t rest until I see Amy’s killer behind bars.’

‘I don’t doubt it.’

‘I heard you’d moved away,’ Kate said, placing a biscuit in her mouth and brushing the crumbs from the table.

A sadness filled his eyes. ‘Things weren’t the same after… we needed a fresh start. My wife’s family live in Devon and her dad was looking to retire from farming; he invited me to take over while he dealt with the business side of things.’

‘You look well for it. All that fresh air suits you.’

He rubbed his hand over the thick stubble covering the lower half of his face. ‘I’m not sure it has a lot to do with the fresh air. It’s been tough trying to focus on the future while Amy’s murder still hangs over us all. I wish there was something I could do to help.’

‘So do I.’

‘You’re certain the man who killed Amy is the same man who killed these other women? Armitage suggested there could be more than one killer?’

‘He’s just trying to cover his own arse. Trust me, when he catches the sole person responsible for all four murders, he’ll realise I was right all along.’

‘You’re very passionate. You’re the only person I’ve ever really believed when they’ve said they were going to catch Amy’s killer.’

‘Good policing is what will finally catch him, not passion.’

‘Listen, I don’t want to cause offence – especially if Armitage is a friend of yours – but it seems like he’s got his own agenda. I don’t know what I’m trying to say… it just sometimes feels like he’s heading up a PR campaign, rather than an investigation. I had confidence in you, but I’m just not convinced that he’ll find Amy’s killer.’

‘That’s kind of you to say, but I’m sure Armitage will do a good job,’ she lied, dipping another biscotti into her coffee. ‘Remind me, when was the last time you saw Amy?’

‘The Saturday before it happened. She asked me to come over to be around while some engineer came to reformat her computer. It had contracted a virus and needed to be repaired. She was good at a lot of things, but not very tech-savvy, or house-proud; her place was a state! The guy showed up, stripped the hard drive back, and reinstalled the operating system and then the software she’d requested. It took a few hours. When Amy returned from work she offered to buy me dinner as a thank you.’

‘You were close, weren’t you?’

He looked away as he nodded, the pain almost unbearable. ‘Yes. I felt protective towards her. I still feel like I failed.’

Kate reached out and took his hand. ‘It wasn’t your fault. Nobody could have predicted what happened.’

‘When I was checking her browsing history I saw she’d signed up to several dating agencies, but it was only after the funeral that I realised they were work-related.’

Kate suddenly looked down at her coffee, conscious she’d said too much. ‘We probably shouldn’t be talking about this.’

‘No, please, I want to know what happened. I mean, what really happened. I read what the papers printed, but there must be more to it than that.’

‘Then you should ask Armitage.’

‘We both know he won’t share any information with me. Please, Kate? I’m desperate to know what really happened to my sister.’

Kate sighed. ‘It was a theory of ours, a loose one but it was all we really had. Each of the three women he’d killed before had featured on one dating site or another. Never the same site, but I felt that was how he identified his victims.’

‘Do you think he ever met any of his victims face-to-face?’

‘He had to have, as he caught them close to their homes. We posted Amy’s profile on several different sites to see what kind of attention she received. You won’t be surprised to hear that she was very popular. We ran background checks on every client who made contact. We searched phone and bank records to see if any were in the locations of the murders at the right time, and ruled out most of them. Where there were those who warranted further investigation – either because we couldn’t find out where they were previously, or they had a chequered past – we arranged for Amy to go on a date with them, while my team sat in a van outside, ready to pounce.’

‘Couldn’t you just demand the lists of clients from those sites the women had been on and try to find the person who had seen all of their profiles?’

‘We tried that, but the problem with the digital age is that anyone can reinvent themselves at any time. We looked for patterns; for similar dates of birth, heights, locations, profile pictures, IP addresses, you name it, but the user numbers are so vast. If he used false information each time, and was logging on from different computers then we’d never find him. I know it’s a cliché, but it was actually like looking for a needle in a haystack.’

‘Can I be blunt?’

Kate finished her coffee. ‘Of course.’

‘Are you absolutely certain that the same man murdered each of those women and Amy? Is it not possible you’re looking for two killers? Or three? Or even four, like Armitage thinks?’

‘I’m certain it’s the same person, and what’s more, I don’t think he stopped. Someone like that doesn’t just stop.’

‘Maybe he was arrested for something else and stopped because he wound up in prison?’

‘The same thought has crossed my mind, but I’m sure the SIO who took over from me reviewed arrest records around the time of Amy’s death, but couldn’t find a viable suspect.’

He checked his watch. ‘I’ve taken up enough of your time, I bet you’ve got a million and one better things to do than keep me company. Just promise me one thing: if there is anything I can do to help you catch him, please ask. I owe Amy that much.’

She offered him the last biscotti as a thought dawned on her. ‘You really want to help?’

He threw the biscuit into his mouth. ‘It would mean a lot.’

‘Is your car nearby? I could do with a lift somewhere.’