As she turned the corner and came face-to-face with her younger sister, she crumpled. Laura’s malignant glare said: ‘Pull yourself together.’ Karen swiftly recovered her sullen insolence. I couldn’t help wondering if Laura had thrown her a similar look that July evening as Marion turned into Sangora Road. Perhaps Laura Foster had been much more than bystander in this murderous escapade.

Shep charged Karen and Laura Foster with the murder of Marion Ryan. As they were escorted to separate cells, they didn’t look at each other, or utter a single word.

Shep summoned the team for a newsflash. He looked strained, thwarted.

‘The good news is, we can now prove beyond doubt that Laura Foster was at the scene of the crime,’ he said.

‘We have established major flaws in both the Foster sisters’ alibis. We can show that Karen had motive: she was sleeping with Marion’s husband Peter until two weeks before her murder. According to her work colleague, Bethan Trott, she’d shown signs of being obsessed with him.

‘Why did they act together? I spoke to our old friend Professor Richards, and he mentioned a condition known as folie à deux. This is a psychiatric term for a shared psychosis or delusional belief between two people who are very close, often twins. He showed me a few examples and the best way I can describe it is “psychosis by osmosis”. Did Karen’s obsession with Peter Ryan somehow rub off on Laura? Did her devotion to Karen suck her into a state of mind where she would kill for her sister? We can get a judge to order psychiatric reports and investigate this right away.

‘Normally, we’d now try to turn the suspects against each other. But as the Prof has pointed out, these girls are too close for that. Both stopped talking to us some time ago.

‘I’ve just come from the lawyer who advises me that we still haven’t got enough to swing a jury. So where do we go from here? According to the pathologist, only a man could have inflicted several of the knife wounds suffered by Marion because of the strength required. Who was it? We’re still matching prints from employees and ex-employees of the Pines care home with those found at the scene, but so far we’re drawing a blank.

‘The bottom line is, until we find the man and can place both him and Karen at the scene, we haven’t got a case. And, to be blunt, I’m clean out of ideas.’

Marion’s butchering of my little finger flashed through my mind so I piped up. ‘The weapon, Guv.’

His tired eyes located me and squinted.

‘What about the weapon, Lynch?’

‘Nothing concrete, Guv. But let’s suppose that the murder weapon was the metal ruler that DS Barratt found in their dad’s mop this week. He told us how nervous Terry got when he was asked about the ruler and whether he’d found it in the mop the next day. We now know that Laura left a gym bag in Bethan Trott’s room. We know she came from the scene of the crime so the ruler would have been in that bag until they picked it up at midday the next day. It definitely wasn’t in the mop the day after the murder and Terry knows this. Also, I checked out Terry Foster like you asked. He’s got form for burglary. It’s twenty years ago, but he did a stretch. Eighteen months.’

‘Get him in,’ barked Shep, ‘and get that fucker Peter Ryan back in. Something’s not adding up about his story.’

Chapter 39

Clapham Police Station, South London

Sunday, August 18, 1991; 16:00

Shep and I sat in the interrogation suite’s viewing gallery, watching Peter Ryan fumble with his wedding ring.

‘We’re letting Terry Foster stew in the other suite,’ said Shep, ‘Barratt told him we’ve got some news about the weapon. By all accounts he’s rattling away in there like a garden gate in a hurricane!’

Yet again, they let Peter bring in his work pager. I couldn’t understand how this object could be considered any less dangerous than his cash, belt or keys. My mind flashed back to the night of Marion’s murder on Sangora Road, Peter fumbling in his pocket for the flat keys, when my mind snagged on a tiny detail.

‘You know, in his statement, Peter said he always kept his keys in his briefcase at work, otherwise he’d lose them. Karen would have known that.’

Shep blinked repeatedly. ‘What are you saying, Lynch?’

‘What if Karen or Laura took the keys out of his briefcase, let themselves into 21 Sangora and waited for Marion?’

Shep nodded slowly.

The sight of Peter’s pager transported me back to last night. Fintan removed the batteries from his so that no one could triangulate his location. I couldn’t fail to suppress a guffaw.

‘What’s so funny, Lynch?’

‘Fintan often takes the batteries out of his pager. He claims that otherwise spooks could work out his location, because the messages are re-directed from local transmitters.’

Shep smiled: ‘He’s got a vivid imagination, that boy.’

We both looked at each other suddenly, thinking the same thing. Karen was a trainee nurse at the Pines. She too would have been issued a work pager as standard. I followed his march to the kitchenette.

‘We need to find out if Karen Foster has a pager. We need to get hold of it and get it to our tech people,’ he gabbled, ‘that way we can place her at Sangora Road on the day of the murder.’

‘Better still, why don’t I just tell lover boy that we already have?’ said Mick.

Shep nodded.

I chased him back to the viewing gallery. He got on the phone to Barratt, told him to track down Karen’s work pager as a priority. He hung up and said: ‘I wouldn’t be surprised if the crafty bitch has already destroyed it. She’s done her homework, that girl.’

Mick and Colin strolled into the suite, smiling and relaxed. Peter frowned in confusion. Mick switched on the tape recorder and Colin took up the slack.

‘We’re having a very good day, Peter,’ he said brightly. ‘We now have evidence that Laura was at the scene of Marion’s murder.’

‘And we now know where you and Karen were at the time Marion died,’ added Mick, planting his pager on the table, ‘thanks to the marvels of modern technology.’

Peter frowned.

‘DS Mulroney has just placed his staff pager on the table,’ Mick told the tape recorder.

‘May I see yours?’ Colin asked.

Peter visibly sagged, unclipped it and slid it across the table.

‘We got hold of Karen’s work pager a few days ago and let the tech boys do their work. Did you know that every pager message is relayed via the nearest transmitter? That means we can work out exactly where a pager has been. I must say, we were a little surprised to see where Karen spent her afternoon.’

Peter’s hideous, hooked-fish face returned.

‘Is there anything you’d like to tell us, Peter?’

Chapter 40

Clapham Police Station, South London

Sunday, August 18, 1991; 17:00

When Shep declared he’d be taking the interview with Terry Foster, neither Mick nor Colin looked remotely surprised.

‘Don’t you mind?’ I asked Mick as we settled in the viewing gallery.

‘Shep’s like the cavalry,’ he quipped, ‘he loves riding in when all the hard work’s been done.’

Terry sat bolt upright, rigid, defensive. Karen had clearly inherited his talent for sullenness. He was a short, slight man, which he compensated for by growing a scrubby beard and holding himself like a bantam cock – chest out, head wobbling, objectionable. His gaunt face radiated ill-health – a heavy smoker for sure. He wore black tracksuit bottoms, white trainers and a skin-tight grey Puma t-shirt which showed off his muscular arms, vein-green from faded tattoos.

‘Shouldn’t we get him a duty solicitor, even if he doesn’t want one?’ I asked. ‘At least then he can’t complain later?’

‘Maybe he’s ready to spill,’ said Colin.