Two days later, Alex and Chloe sat opposite Isobel Matthews in one of the station’s interview rooms. This time, she had been formally arrested. By the end of the interview, Alex thought, as she pressed a button on the tape recorder, the girl would be charged with two counts of murder.

‘You’ve had quite an ordeal these past few days,’ she noted, after Isobel had confirmed her name and address for the recording. The girl said nothing. They were back to this; the silent treatment and the blank stares that Isobel obviously hoped would be enough to save her skin.

But not this time.

‘Where were you on the evening of Sunday 11th June?’ Alex asked.

Isobel Matthews glanced at her solicitor. She didn’t look as self-assured as she had during the last interview, as though she knew that this time the police had secured the evidence they had previously been lacking.

‘I don’t know,’ she answered. ‘At home, probably.’

‘Would anyone else be able to confirm that?’

‘I live alone.’

Alex nodded. ‘Nice flat for someone your age. Cardiff Bay, overlooking the water … must have cost a fair bit. So let’s get this right. You hate your father for what he’s put your family through over the past few years, that much is obvious. The affairs with his students, the allegations of sexual harassment – that must all have been pretty distressing for you, as well as embarrassing. But you were happy enough to have him buy you a flat.’

Isobel pursed her lips. ‘It was a deposit,’ she corrected. ‘Plenty of people get help from their parents with that. And it was my mother’s money anyway.’

Isobel Matthews – along with her mother, Melissa, and younger sister, Olivia – now knew about the affair her father had had when Melissa had been pregnant with her first child. They knew of Leah’s deception, and also of the fact that a real half-sister existed: Kirsty Brooks, who maintained she wanted nothing to do with Leighton or his family. Kirsty was now at university in Reading. The news of Leah’s deception had apparently come as little surprise to her, though it must have smarted to realise how she had been used and manipulated by a girl she had regarded as a friend and trusted with her secrets.

By all accounts, Melissa had finally told Leighton to leave. Wherever he was staying, he had decided to keep a low profile. Alex had been half expecting him to turn up at the station in a bid to find someone else to blame for his new-found homeless status, but perhaps he was finally taking responsibility for his decisions and actions.

Although she doubted it.

‘Had you ever been to that student house before the night of the party?’ she asked.

‘I’ve never been to that house,’ Isobel said. ‘Not on the night of the party … not ever before it.’

‘Other than the night you went to Railway Terrace with the intention of hitting Leah with your father’s car,’ Chloe said.

‘What’s being done about that man?’ Isobel asked, ignoring Chloe’s comment. ‘I get hit on the head, abducted, nearly set fire to, but I’m the one here being questioned.’

‘It was water,’ Alex said, sitting back and folding her arms across her chest.

‘What?’

‘The petrol can. He covered the car in water. He wasn’t going to set fire to anything, Isobel; he just wanted you to think you were going to suffer. He wanted you to know what fear feels like.’

Alex sat forward again and reached for the file that sat on the desk in front of her, retrieving a photograph from its contents. Jamie Bateman had been charged with assault and abduction; Alex wished the outcome could have been different for him.

‘Do you recognise this?’ she asked Isobel, pushing the photograph across the table.

Isobel glanced briefly at it. ‘No.’

‘You didn’t take a very close look.’

‘I don’t need to. I don’t recognise it.’

Alex slipped a hand into the file again and produced another photograph. This time, Isobel’s reaction was obvious. It was a photograph of her taken on a night out with friends; an old profile photo retrieved from her Facebook page just the previous day.

Alex turned the image so that it was facing Isobel. She placed a finger on it, pointing out the necklace hanging at Isobel’s throat. ‘Doesn’t the charm on the necklace you’re wearing there look very much like the one in the first photograph I showed you?’

Isobel shifted in her seat. ‘So? There’s probably loads of them about.’

‘I notice you’re wearing a similar gold chain today. No charm hanging from it, though.’

‘It’s never had one.’

‘When you provide us with that chain you’re wearing at the end of this interview, Isobel, a professional jeweller will be able to identify whether this charm,’ Alex gestured to the first image she had shown her, ‘is a match and was indeed once part of that necklace. Gold has identifying features … it’s not a difficult process, apparently.’

Isobel’s expression had changed, the colour draining from her cheeks. Her eyes looked grey with the bruising that still remained from Leah’s beating; that greyness was now beginning to stain the rest of her features.

‘This charm,’ Alex said, tapping a finger on the photograph, ‘was found by Keira’s parents when they went to the house to clear her things from her bedroom. It was lying by the skirting board just inside the bedroom doorway. Keira didn’t wear gold jewellery; it irritated her skin. Her parents assumed it belonged to Leah and left it on the chest of drawers. It belongs to you, doesn’t it, Isobel?’

The young woman said nothing. She didn’t bother now to look at the solicitor, who had fallen silent in the seat beside her.

‘One of the other people who attended the party that night has identified you as being there,’ Alex continued. ‘She said she saw you on the first-floor landing not long before Keira fell. What do you think … has this person made a mistake?’

For the first time, there was a reaction. A single tear was now tracking its way down Isobel’s cheek. As with Leah Cross, Alex doubted whether the tear was for Keira.

‘Perhaps this is a mistake as well.’ Alex placed two clear bags on the desk between them. ‘For the purposes of the recording, I’m showing Miss Matthews exhibit N17 and exhibit T12. You recognise both these, Isobel?’

The young woman’s face became ashen as she stared at the knives encased in the plastic bags. She must have known that evidence against her would be found. It seemed the deeper she had fallen into her mission for revenge, the more careless she had become.

‘This knife,’ Alex said, gesturing to the first of the two bags, ‘is the one Leah Cross used to stab herself in the stomach with. The knife that you took with you to the house on Broadway.’

Isobel said nothing, her attention still fixed on the desk in front of her. The solicitor was looking at her with increasing doubt.

‘This,’ Alex continued, pointing to the second bag, ‘is the knife that was used to kill Tom Stoddard. You know who he is, don’t you?’

‘No,’ Isobel said quickly.

‘These knives are part of the same set. They were bought at a DIY shop in Cardiff. Guess whose debit card was used?’

For the first time, Isobel dragged her attention from the knives. She focused instead on her hands in her lap; anything to avoid Alex’s penetrating stare pressing upon her from the other side of the desk.

‘Did you think your father would end up taking the blame for this as well?’ Chloe asked. ‘The store has provided CCTV footage from its car park, Isobel. There’s footage of you leaving the shop after purchasing the knives.’

Alex reached into the file and took a photograph from it: a still from a CCTV camera on the main road four streets from Railway Terrace. She placed it beside the two evidence bags on the table and waited for some sort of reaction from Isobel. DC Jake Sullivan had proven his worth finding this one.

‘Something wasn’t adding up. We know you killed Tom, but we couldn’t work out why. This,’ she said, tapping the photograph, ‘might explain everything. Again, for the purposes of the tape, I’m showing Miss Matthews exhibit T20. This is Tom Stoddard, walking south along Broadway in Treforest shortly before midnight on the same evening on which Leah Cross was hit by your father’s car on Railway Terrace. It would have taken him approximately three minutes to get home from here, by which time it’s very likely he would have met up with Leah Cross, who was also making her way home. He saw you hit her, didn’t he? Or at the very least, you feared he might have? Tom needed to be silenced.’

Isobel remained silent, a fresh stream of tears now falling. The self-pity was strangely reminiscent of Leah Cross.

‘Isobel Matthews. I’m arresting you for the murders of Keira North and Tom Stoddard. You do not have to say anything …’

Alex watched the girl as she cried, unable to feel anything towards her but revulsion. ‘Your bitterness towards your father has taken two lives,’ she told her. ‘Three, in fact. I assume you’ve not forgotten Keira’s daughter?’ The scraping of her chair pierced the uncomfortable silence as Alex stood. ‘It’s cost you your future too, Isobel.’