But there was a sweeter incentive for me to keep going with this; vindication. She’d scoffed at my theory that Marion Ryan’s spirit wanted to help me nail her killer. Now I’d been brought onto the squad investigating her murder, one thing felt certain: I’d go back to 21 Sangora Road.

I’d attended the crime scene on two previous occasions. Both times Marion’s ghost/spirit had launched deranged assaults upon me. What if it happened again? How would Lilian explain away a third encounter? ‘Highly improbable’, indeed.

I don’t believe in ghosts, spirits, the afterlife or any of that stuff. But what Lilian had singularly refused to appreciate was how real these encounters had been.

‘You’re on,’ I said.

Chapter 16

Clapham Police Station, South London

Thursday, August 8, 1991; 11:45

Although deserted, the incident room hummed with subdued intent. Barely larger than an average sitting room, it was crammed with phones and fat computer terminals. Stained coffee cups jockeyed for space with full ashtrays and used sandwich wrappers.

And to think they call us pigs.

Whiteboards lined the walls, listing the names of investigating officers and the computer codes of what leads they were chasing up. Nearby, a large picture map of London was dotted with yellow Post-its.

There was no sign of anyone to set me up, so I headed to the canteen. The overwhelming stench of fried pig had the curious effect of making me both hungry and queasy. I settled for a mug of Nescafé Rough Blend coffee and a can of cola, hoping the combined hit would crank me into first.

I returned to the incident room and took a good look at that note-spattered map of London.

I heard a noise and turned to see a chubby man in his forties feeding disks into one of the fat computers.

‘Hi,’ I said walking over, ‘I’m Donal Lynch, Acting DC.’

He nodded glumly without looking up.

After about ten minutes, he said: ‘I trust you’ve done this before?’

‘What, use a computer?’

‘No, fly a 747,’ he spat, now glaring up at me.

Was this some sort of test?

‘I haven’t used a computer since school,’ I said flatly.

‘Well it’s not my job to teach you,’ he said, getting to his feet and flouncing off.

I typed tentatively. Soon I found myself chewing through megabytes of Space Invader green text. Every investigative ‘action’ had been logged, numbered and described. Separate folders contained statements and reports. I knew exactly where I wanted to start.

During her nocturnal burglaries of my mind, Marion had repeatedly slammed doors. Her message, if I believed in her, had been clear: a door somewhere holds vital evidence to the identity of her killer. Could be a house door, could be a car door – but a door was where I’d find the smoking gun clue.

I clicked open the forensics folder, then the fingerprints file and sought out anything connected to doors at 21 Sangora Road.

I knew from my training that fingerprints are a lottery. A good print on the right surface can last for years. In one celebrated case, forensics dated a single print back forty years. Somewhat perversely, dating fresh prints is more difficult. There are so many factors to take into consideration: the surface the print is left on, whether it has been made with sweat or other substances, the air temperature.

The report said they found fresh fingerprints belonging to Marion’s husband Peter Ryan and his colleague Karen Foster on the front door and on the connecting door into the flat. All they could say with certainty was that these prints were less than twelve hours old. In other words, the impressions had been made sometime between ten a.m. and ten p.m. on the day of Marion’s murder.

They’d found lots of other prints left within the same time frame: sixteen sets on the front door and seven on the internal door to the flat. This seemed a surprisingly large number to me. I thought about the front door to our flat: there’d be no more than three or four different fingerprints planted there on any given day. The front door to the entire block would be plastered with them: ours was one of forty-eight flats. Marion and Peter’s flat was one of just two behind that front door. Yet they’d found sixteen sets of day-old prints on the communal front door. I made a note to investigate the frantic comings-and-goings at number 21 on the day of Marion’s murder.

I then read that they’d cross-referenced all of these prints with police records: none of them matched a known offender.

That was that. The killer’s prints could be on both doors, we just didn’t know who they belonged to. Or Marion’s husband Peter was the killer. Maybe that’s what Marion had been telling me during her ghostly visitations: look no further than him. But unless a witness saw Peter at 21 Sangora Road at around six p.m. on the day of the murder, his prints on the doors were evidentially useless.

I felt flat and a little foolish. Naïvely, I thought Marion had been leading me directly to her killer. But I refused to give up on the idea that a door somehow held the key. I just needed to work out which door, and how.

An overview report in the forensic folder directed me to a filing cabinet containing the scene of crime photos.

These included a close-up of Marion’s torn, dangling fingernail. I couldn’t understand why this single image flooded my body with such leaden sadness. I thought of Marion as a baby; how her mum and dad would have marvelled at this hand and treasured it. And as she grew up, they would have grasped it tightly to cross the road. Her dad held this hand when he gave her away to Peter, probably thinking he’d done his job, that she was safe now.

I got back to the computer to track down the pathology report. I felt under real pressure to find something – anything – that pointed to Peter Ryan. Shep had given me this golden opportunity: I didn’t want to let him down.

Bruising showed that Marion had received a karate-style chop between her upper lip and nose which may have momentarily stunned her. The forty-nine stab wounds were delivered in rapid succession; seven to her chest, nine to her throat, the rest to her lower stomach, back and bottom. One wound pierced her right hand as she tried to ward off the blows. Some were delivered with great force – suggesting a male attacker – but others were mere pinpricks. Could it have been a two-hander? Peter and his secret lover? What about Karen, the colleague who’d been with Peter when he found the body?

The two ‘significant wounds’ were one to her throat which slashed her windpipe and one to the left side of her back which pierced her lung. According to the pathologist, the force of these blows suggested they had been delivered by a man. She died through a combination of loss of blood and asphyxiation as it flowed into her lungs.

The pathologist estimated that the attack would have lasted between two and three minutes. The murder weapon appeared to be a five-inch blade. There had been no sexual assault or wounds to her genitalia. I felt these last facts warranted attention. Presumably, the Lone Wolf Killer had to be some sort of sexual deviant. Didn’t the lack of sexual assault or genital injury point more towards a domestic? The pathologist didn’t say so.

He put her time of death at six p.m., with an hour window each side. A handwritten note said: ‘CCTV provides more precise timing.’ The pathologist noted that he’d never come across a domestic murder victim with more than nine stab wounds. One nil to the Lone Wolf Killer theory.

I took the pathologist’s cue and tracked down the CCTV tapes. I hunted and found a VHS player in a tiny room, closed the blinds and watched with horrible fascination as Marion Ryan strode purposefully to her death.

She left her place of work – Reuters news agency on Fleet Street, where she’d been employed as a copy taker – at 17.06 p.m.