Carmella’s flat was as immaculate and homely as Patrick had always imagined – the home of a couple who obviously had no children. Patrick took a seat at the small table in the living room where, he imagined, Carmella and Jenny ate dinner together while listening to tasteful music. He didn’t imagine them as the types to scoff dinner in front of the TV with plates on their laps, and certainly not at a table with toddler-flung spaghetti shapes and sausages around their feet, CBeebies blaring in the background.

Carmella grabbed her laptop and sat down beside him. ‘Jenny’s at work. She just texted me to tell me she’s got a raging hangover. Apparently, she, Gill and Suzanne’s husband had a good chat after we left your party.’

‘Oh God.’

Carmella chuckled. ‘Don’t worry. Nobody discussed how you’ve got the hots for the guv.’

‘Carmella! I don’t—’

She held up a hand. ‘It’s all right, Pat. I’m only teasing you. But you’ve gone very pink.’

He fixed his attention on the laptop screen. ‘Can we concentrate on this?’

‘Sure.’ The smile slipped from her face and he felt yet another prick of guilt – a sensation he shook off as he watched Carmella type in the URL of the official OnTarget forum. Wendy had told Patrick she had spent most of her time on this site because, although there were plenty of others, this was the most active. Immediately, Patrick realised this was going to be like searching for the proverbial needle. There were thousands of posts, most of them seemingly nonsensical – a sea of acronyms and bouncing smileys.

‘We need to know what her username was,’ Carmella said. ‘Otherwise we’ve got no chance of figuring out who she was chatting to.’

‘I should have got her to tell me.’

‘What about Strong’s team? They must have figured it out already. Can’t we ask them? We are meant to be working together, after all.’

Patrick shook his head. He knew that would be the sensible thing to do, but he was paranoid about Strong trying to take over the entire investigation, especially if he admitted to any weakness. That weakness being that, so far, they didn’t have a bloody clue who had murdered Rose and Jess, despite having worked on this investigation for a week and a half.

‘No. Let’s try to figure it out ourselves first.’

She looked at him, then nodded. ‘OK. We know Wendy went to the book signing at Waterstones – I saw her there – so maybe she was involved in one of the chats about that.’

Carmella typed ‘waterstones’ into the search box and two dozen forum topics appeared on screen. She sighed and began to click on each one in turn, skimming through the discussions about the event, from the build-up, with all the fizzing excitement about being in the same room as the OnT boys, through to the aftermath, with loads of links to photos of the signing, dozens of selfies with the pop stars behind a desk in the background. Patrick glanced over the photos to see if he could spot Wendy – he couldn’t – but that wouldn’t be helpful anyway.

‘Look,’ he pointed out. ‘There’s a number beneath each name stating how many posts they’ve made.’

Most of them numbered in the hundreds or thousands. Blake7 – 2,356 posts; CroydonChick – 1,398 posts; Jade – 18,467 posts.

‘Good grief!’ Patrick exclaimed. ‘I wonder if I’d have used these forums if they’d been around when I was a teenager.’

‘Yeah, in those days you had to use smoke signals, didn’t you?’

Patrick smiled but wasn’t in the mood for banter. ‘Look, this one, ShawnsCupcake, has only posted seventy-four times.’ He tapped the screen, indicating a message about the book signing: ShawnsCupcake asking who else was going to be there.

‘Let’s have a look at her profile,’ Carmella said.

Clicking on the username took them to a new screen showing the profile of ShawnsCupcake. The profile picture was, like many of those on the forum, a photo of Shawn, giving nothing away about the real identity of the user. Again, Patrick wished dearly that he’d got more detail from Wendy about what she was doing. He hadn’t realised there would be a time limit. But he still blamed himself, knew he wouldn’t stop beating himself up about it until he’d found her murderer. And even then, he didn’t know if he’d feel better. Because whatever happened, poor Wendy wasn’t coming back. She would never achieve the potential he knew she’d had.

‘ShawnsCupcake joined on the eleventh of February,’ Carmella said, snapping him out of his reverie. ‘Is that the date she started?’

Four days ago. ‘Yes, that’s right.’

The page linked to all of the discussions ShawnsCupcake had taken part in. The first was a joke about Shawn making her feel like a Haribo sweet.

‘Does that sound like something Wendy would write?’ Carmella asked.

‘I think so.’ He thought about the message in the Valentine’s card. You make me melt like chocolate. Carmella didn’t know about the card yet, but he bet it wouldn’t be long before word got around the station.

‘Look,’ Carmella said. ‘There’s a discussion here about football – did you know that Carl from the band is rumoured to be buying his local team, Torquay United?’

In the discussion, most of the users were talking about how they were going to become Torquay fans, that they were going to start going to the matches, despite agreeing that most of them hated football.

ShawnsCupcake had written, Not me! I’ll be Wolves till I die. Even though I live down south in Kingston. ☺.

Wolves. Wolverhampton Wanderers. Wendy’s hometown team. And she’d lived in Kingston.

‘It’s definitely her,’ Patrick said, sitting up straighter.

‘Look at this. She started a thread about the murders: I have a theory about what happened to them but I’m too scared to share it on here. Fuck. Looks like she was trying to flush out anyone with information.’

‘And it worked. Can we access her private messages?’

She gave him the look she used when he said something that made him sound like an old man. ‘Not without her password.’

‘Yeah, I knew that . . .’

‘And we could sit here typing in educated guesses all day, but we’re unlikely to get it right. This isn’t one of those stupid films. We need to talk to Strong’s team. They’ve got her computer – they’re bound to have found all her log-ins.’

‘And I’ll ask Graham Burns. You know, the social media guy. He gave me the messages that Rose and Jess exchanged.’

Patrick stood up and walked away from the table, over towards the window. He looked down at the street, red buses gliding by, a cyclist weaving through the traffic.

‘If it was you . . . if you were the person who’d killed Rose and Jess – assuming of course that you use the forums, which you probably do, to have found them – and you saw that, what would you do?’ he asked. ‘You’d want to know if this theory bore any relation to the truth.’

‘Yes, and I’d private message her. Find out about this theory.’

Patrick stepped away from the window. ‘The way Wendy was killed was completely different to Rose and Jess. Nancy Marr too. No sign of torture, just a swift . . . execution.’ He winced, imagining the shock Wendy must have felt as the knife flashed in the darkness.

‘He was trying to keep her quiet. Stop her exposing him.’

‘Which suggests that Wendy actually did have a theory, and that it was close to the truth. Close enough to worry the killer, anyway. We really need to get into her private messages. Let me call Burns now.’

He had Burns’s number stored on his phone. Burns picked up on the third ring and Patrick explained what he needed. ‘All private messages sent and received by a user called ShawnsCupcake.’

Burns made a groaning noise. ‘You know I could get in a lot of trouble for this . . .’

‘A police officer was murdered and we believe it was by someone using your forum. Now, if you want the whole OnTarget website shut down, your computers impounded, while we—’