The murder of an Afghan shopkeeper at Stroud Green, attacked by a gang of youths armed with blades and iron bars, beaten and left for dead, was foundering amidst a welter of denial, false alibis and barefaced lies. The two fourteen-year-olds they were certain had been responsible for setting fire to an eighty-six-year-old woman after breaking into her flat, had been arrested and then grudgingly released for lack of evidence. The week before, a family in Wembley, a mother and three children under ten, had been found bludgeoned to death, two of the children in their beds, one on the stairs, the mother in the garden as she tried to raise the alarm. The father had hanged himself from the top of a brightly coloured climbing frame in the kids' playground of the local park.

And then there were the young black men: investigations undertaken with DCC4, Racial and Violent Crimes. One man shot dead as he sat drinking coffee at a pavement cafe in Camden Town; another, possibly as a reprisal, gunned down as he came up the steps from Willesden Green station; a third, no more than seventeen, knifed outside the bowling alley in Finsbury Park. On and on.

Karen knew the figures: the murder rate in England and Wales for the previous year was the highest ever, with shooting-related deaths up by some thirty-two per cent. The highest overall recorded crime rate was in Nottinghamshire, though violent crime, per head of the population, was more prevalent in London, with men under the age of twenty-six the most frequent victims. Gun crime aside, the biggest increases were in stranger violence, harassment and rape. And despite the growing prevalence of guns on the streets, the most popular murder weapon by far was still some form of sharp implement. Knife. Machete. Razor. Sharpened spade.

She thought of this as, having turned her car around, she fought it back through the rush-hour traffic; single men in suits steering one-handed as they smoked cigarette after cigarette and snapped, illegally, into their mobile phones; smart young mums ferrying their children to school in SUVs.

'When you goin' to settle down, girl?' her grandmother had asked when she made her last visit home. 'Have some babies of your own?'

Home was Spanish Town in Jamaica, the progeny of sisters and cousins swarming round her like an accusation.

'Girl, you not gettin' any younger.' As if, not so many months off thirty-nine, she needed reminding.

At Crouch End Broadway, Karen steered wide past a car hesitating at the pedestrian lights, slid into the left-hand lane and accelerated up the hill. Incongruous, a giant totem pole outside the playground signalled the entrance to the lane, and she slowed almost to a halt before pulling in behind Mike Ramsden's Sierra.

A quick glance in the rear-view mirror, a hand pushed up through her tousled, short-cut hair; by rights her lipstick could do with replenishing, but for now it would have to do. She was wearing a dark brown trouser suit and boots with a solid heel that brought her as close as damn it to six foot. Well, five ten. Her don't-mess-with-me look, as she liked to think.

Removing his hands from his pockets, Ramsden walked towards her. Down below, she could see Forensics already at work, shielding the body from sight.

'What have we got?' Karen said.

Ramsden coughed into the back of his hand. 'White female, thirty-five to forty-five, multiple stab wounds; dead some little time. Last night at a guess.'

'ME not here yet?'

'Stuck in traffic'

'Tell me about it.' Karen moved closer to the edge and looked down. 'That where it happened?'

'My guess, she was attacked somewhere up here and then pushed.'

Karen looked along the area to their left that had now been cordoned off, the muddied slope leading steeply down.

'Marks you can see,' Ramsden said. 'That and the angle of the body.' He shrugged. 'Maybe he finished her off down there, who knows?'

'Any ID yet?'

'Not so far.'

'No one similar reported missing?'

'Early days.'

Karen sighed and patted her coat pocket, hoping for a mint; since she'd stopped smoking on New Year's Day, she'd been committing dental suicide.

'Any idea yet what she was doing here?'

Ramsden told her so far they'd found a grey sports bra and matching vest, the vest dark with mud and what was almost certainly blood. A pair of grey jogging pants had lain nearby. One blue-and-white Puma running shoe had been discovered close to the body, the other amongst the trees at the far side of the old railway track, where presumably it had been hurled.

'Out running,' Karen said. 'Chances are she'll live close.'

Earlier in the year a woman had been attacked and killed while jogging in east London, Hackney. Stabbed. The investigation was still ongoing.

Karen glanced round at the flats that ranged below. At the end of the lane, she knew, a path led down to a crescent of Victorian houses and the sprawl of another low-rise council estate at the far side of Hornsey Road. Before being assigned to SCDl, she'd run a missing-person investigation here, a three-year-old boy who'd gone missing from the nursery and been found forty-eight hours later, safe but cold, asleep in someone's garden shed.

'Who found the body?'

Ramsden pointed towards a thirtyish woman in a yellow Puffa jacket, standing with two others of similar age. All with cigarettes on the go.

'Who talked to her?'

'Furness and Denison.'

'Talk to her again.'

'But…'

'Again, Mike. Do it yourself. I'm going down to take a look.'

Her protective clothing was in the boot of the car. Changed, she made her way carefully down, not wanting to make a fool of herself by slipping. The DI in charge of the Forensic team was someone she'd worked with before.

Inside the canopy, Karen bent towards the body. Some of the cuts looked superficial, others, she guessed, ran deep. There was bruising to the neck and face, another bruise – the result of a kick? – above the pelvis on the left-hand side. A fine spray of dried blood speckled the inner thigh, and something silver and crystalline trailed, snail-like, across the curve of her stomach.

Sexual assault?

Until the post-mortem there was no way to know for sure.

She stepped back outside and turned in a slow circle, trying to get a sense of what had happened, taking her time.

Ramsden was on his way towards her, having taken the long way round.

'The woman,' Ramsden said. 'Nothing she didn't say first time round.' He took a stick of chewing gum from his top pocket, removed the wrapping and put it in his mouth.

Karen held out her hand.

'Sorry,' Ramsden said. 'Last one.'

She didn't know whether to believe him or not.

'She recognise the victim?' Karen asked.

'Not from what she saw.'

'Get her to look at one of the Polaroids. Good chance, if they both use this place a lot, she'll have seen her before.'

But now Denison was shouting something from above, altar-boy face shining and a canvas sports bag held high in one gloved hand.

'Lucky bollocks,' Ramsden said, half beneath his breath. 'Fall in shit and he'd come up with a five-pound note.'

They climbed back up.

'It was there,' Denison said, pointing. 'Community centre. Pushed down below the steps by the door.'

'You've checked inside?' Karen asked.

Denison shook his head. 'Just a quick look. Sweatshirt. Towel. Socks.'

'Then we don't know it's hers,' Ramsden said.

'Let's see,' Karen said, reaching into the bag with gloved hands.

The wallet was safe in an inner pocket, square and dark, the leather soft with use. She lifted it out and let it fall open in her hand.

'Oh, shit,' she said softly. 'Shit, shit, shit.'

'What?' Ramsden said.

Karen held out towards him the warrant card with its small square photograph: Maddy Birch, Detective Sergeant, CID.