They walked on, turning left on to Aerodrome Road and falling into step within a few paces.

‘I think you were right the other night,’ Thorne said. ‘It’s somebody she knew. Not well… not necessarily, anyway. Maybe he works in a local shop, does next-door’s garden, whatever.’

‘A face she recognises.’

‘That’s all he needs to be. You heard what Walker said about if it had been a different day. Sounds like whoever killed Emily had been watching, and for a while. He knew their movements, knew when the time was right.’

‘So he targeted her?’

‘Looks that way. He wasn’t just ringing doorbells until someone answered that he liked the look of.’

‘Why Emily, though?’ Holland asked.

Thorne looked sideways at him and Holland acknowledged the stupidity of asking the question now, when they had so little to go on. When there were a thousand answers, and none at all. They both knew that the true answer, if they ever found it, would almost certainly give them their best chance of catching whoever had killed Emily Walker. At that moment, Thorne could do no better than a muttered ‘Christ knows’, before jogging across the road and walking quickly towards the main gate.

‘That’s weird though, isn’t it, this Garvey business?’ Holland was doing his best to keep up, a few feet behind Thorne. ‘Before my time, but shit… that was a big case, wasn’t it?’

Ahead of him, Thorne was waving his ID at the officer inside the control box.

‘Did you work on it?’

Half a minute later, it was Holland ’s turn to wait, light rain blowing into his face, while his warrant card was checked. Thorne was already twenty feet clear of the barrier and moving across the car-park towards Becke House. He didn’t appear to have heard Holland ’s question.

Thorne had worked on the Raymond Garvey investigation, though not in any significant way. He’d knocked on a few doors, been part of a fingertip-search team one night. At the time, it was the biggest investigation for a decade or more, with hundreds of detectives working to catch a man who would eventually murder seven women. There can’t have been too many officers in the Met who had not been involved in some capacity.

Inside Becke House, Thorne walked into the lift and jabbed the button for the third floor, thinking back.

He was an up-the-sergeant’s-arse, eager-to-please detective constable back then. Kentish Town CID, the station no more than five minutes’ walk from where he lived now.

The lift doors were stubbornly refusing to close, so Thorne stabbed at the button again. He was ashamed that he could remember every detail of a blue suit he used to wear back then and the number plate of the car he’d been driving around in, but not the names of Raymond Garvey’s victims.

The door finally slid shut.

Not a single one…

He told himself that it was always the way, especially with a series of killings. How many of Dennis Nilsen’s fifteen victims could he name, or Colin Ireland’s five? Could he remember any of Harold Shipman’s two hundred or more?

Out of the lift, he walked down the corridor, past the Major Incident Room and towards the small office he shared with DI Yvonne Kitson.

It was different with his own cases, of course. He could remember every name, every face; each ‘before’ and ‘after’ photograph. Her mother’s name might not have been as instantly familiar as it should have been, but Thorne knew he would never forget Emily Walker’s.

Kitson had left a note on his desk about a case that was due in court the following week and some evidence that needed chasing up. Thorne laid it to one side and pulled the computer keyboard towards him. All the way back from Colindale, he had been wondering where the Garvey case notes would have been archived. Now, he decided there was a far quicker way to do a bit of research.

Thorne hit a few keys and logged on to Google. Typed in ‘Raymond Garvey’.

There were over three hundred and fifty thousand hits.

He scrolled past the first half a dozen links, ignoring Wikipedia and something called serialkiller.com, until he found a site that was not advertising a magazine or true-crime shows on satellite TV and seemed more or less reliable. Hee looked at the list of names. Susan Sharpe, aged forty-four, was number four. She had been attacked on her way home from a gym, bludgeoned to death, as had all the other victims, and been found on a canal bank in Kensal Green, the vast mausoleums and elaborate statuary of its famous cemetery spread out alongside. Thorne clicked on the name and brought up a picture. He saw no immediate resemblance to Emily Walker, then reminded himself that he had never seen Emily alive.

Raymond Anthony Garvey had murdered seven women in four months. He might have killed many more had he not been arrested after a simple pub brawl in Finsbury Park. Had a sample of his DNA taken after that incident not matched that found on two of the victims. It was the kind of coincidence that would have crime-fiction writers accused of laziness, but good luck played a bigger part in cracking such cases than most senior police officers would care to admit.

Garvey, who always refused to talk about his motives, was given five consecutive life sentences, and was told by the judge that he would die in prison. That happened a lot sooner than anyone expected, as he was diagnosed with a brain tumour twelve years into his sentence and succumbed to it six months later.

Thorne looked again at the picture of Raymond Garvey – the bland, blissful stare of an ordinary psychopath – before highlighting the names of the women he had murdered. Just after he’d clicked PRINT, the door opened and Russell Brigstocke walked in.

The DCI dropped his sizeable backside on to the edge of Thorne’s desk and glanced at the images on the computer screen. He nudged at his glasses. ‘ Holland told me about that. What are the bloody chances?’ He pushed his fingers through what had once been a pretty impressive quiff, but was now getting decidedly thin.

‘Yeah.’ Thorne knew that his own appearance had changed just as much. There was still more grey hair on one side than the other, but a lot more of it everywhere. He logged out of the website, Garvey’s face giving way to a blue screen and a Met Police logo: the reassuring words ‘Working Together for a Safer London’.

‘Thirty-six hours into this one already, Tom,’ Brigstocke said. ‘Where are we?’

The DCI could interpret Tom Thorne’s expressions and his curt body language as well as anyone. He recognised the twitch in the shoulder that meant ‘Nowhere.’ The puff of the cheeks that said, ‘Barring our killer handing himself in, you won’t be standing outside Colindale station making triumphant announcements to the press anytime soon.’

‘What’s happening with the FSS?’ Thorne asked.

The Forensic Science Service lab in Victoria was busy examining all the trace evidence gathered from the crime scene: hairs, fibres, fingerprints. They were analysing the bloodstain pattern in the hope of creating an accurate reconstruction of the crime. They were trying to identify the fragment of celluloid found clutched in Emily Walker’s hand.

‘I’m chasing,’ Brigstocke said. ‘Same as I always am. Tomorrow, with a following wind, but more likely Sunday.’

‘What about the E-fit?’

‘Have you seen it?’

Thorne nodded. The curtain-twitching neighbour had clearly not witnessed as much, or in as much detail, as he had first claimed. ‘I’m not holding my breath,’ he said.

‘Right. I don’t think it’s going to help us a great deal either, but what do I know? Jesmond wanted it out there on the hurry-up, so it’s out. It’s in the Standard today, and some of the nationals. London Tonight, too.’

Brigstocke was every bit as transparent as Thorne himself, and Thorne caught the roll of the eyes that translated as, ‘Waste of fucking time.’ Of course, Superintendent Trevor Jesmond would want the E-fit distributed as widely as possible, to show that his team were making progress. It did not seem to concern him as much as it should – with a picture of the killer that looked as though it had been drawn by a chimpanzee – that precious time and manpower would now be wasted taking, logging and filing hundreds of pointless calls, mental or plain misguided, proclaiming that the person the police were looking for was everyone from the man next door to Johnny Depp.