Joona Linna.

Stalker.

Lars Kepler.

It wasn't until the first body was found that anyone took the film seriously. A link to a video clip on YouTube had been sent to the public email address of the National Criminal Investigation Department. The email contained no message, and the sender was impossible to trace. The police administration secretary did her job, followed the link, watched the film, and a.s.sumed it was a rather baffling joke, but nonetheless entered it in the records.

Two days later three experienced detectives gathered in a small room on the eighth floor of National Crime headquarters in Stockholm, as a result of that very film. The oldest of the three men was sitting on a creaking office chair while the other two stood behind him.

The clip they were watching on the wide computer monitor was only fifty-two seconds long.

The shaky footage, filmed in secret on a handheld camera through her bedroom window, showed a woman in her thirties putting on a pair of black tights.

The three men at National Crime watched the woman's peculiar movements in embarra.s.sed silence.

To get the tights to sit comfortably she took long strides over imaginary obstacles and did several squats with her legs wide apart.

On Monday morning the woman had been found in the kitchen of a terraced house on the island of Liding, on the outskirts of Stockholm. She was sitting on the floor with her mouth grotesquely split open. Blood had splattered the window and the white orchid in its pot. She was wearing nothing but a pair of tights and a bra.

The forensic post-mortem later that week concluded that she bled to death as a result of the multiple lacerations and stab-wounds that were concentrated, in a display of extraordinary brutality, around her throat and face.

The word stalker has existed since the early 1700s. In those days it meant a tracker or poacher.

In 1921 the French psychiatrist de Clerambault published a study of a patient suffering from erotomania. This case is widely regarded as the first modern a.n.a.lysis of a stalker. Today a stalker is someone who suffers from obsessive fixation disorder, an unhealthy obsession with monitoring another individual's activities.

Almost 10 per cent of the population will be subjected to some form of stalking in the course of their lifetime.

The most common form is when the stalker has or used to have a relationship with the victim, but in a striking number of cases when the fixation is focused on strangers or people in the public eye, coincidence is a key factor.

Even though the vast majority of cases never require intervention, the police treat the phenomenon seriously because the pathological obsessiveness of a stalker brings with it a self-generating potential for danger. Just as rolling clouds between areas of high and low pressure during stormy weather can suddenly change and turn into a tornado, a stalker's emotional lurches between worship and hatred can suddenly become extremely violent.

1.

It's quarter to nine on Friday, 22 August. After the magical sunsets and light nights of high summer, darkness is encroaching with surprising speed. It's already dark outside the gla.s.s atrium of the National Police Authority.

Margot Silverman gets out of the lift and walks towards the security doors in the foyer. She's wearing a black wrap cardigan, a white blouse that fits tightly at the chest, and high-waisted black trousers that stretch across her expanding stomach.

She makes her way without hurrying towards the revolving doors in the gla.s.s wall. The guard sits behind the wooden counter with his eyes on a screen. Surveillance cameras monitor every section of the large complex round the clock.

Margot's hair is the colour of pale, polished birchwood, and is pulled into a thick plait down her back. She is thirty-six years old and pregnant for the third time, glowing, with moist eyes and rosy cheeks.

She's heading home after a long working week. She's worked overtime every day, and has received two warnings for pushing herself too hard.

She is the National Police Authority's new expert on serial killers, spree killers and stalkers. The murder of Maria Carlsson is the first case she's been in charge of since her appointment as detective superintendent.

There are no witnesses and no suspects. The victim was single, had no children, worked as a product advisor for Ikea, and had taken on her parents' unmortgaged terraced house after her father died and her mother went into care.

Maria usually travelled to work with a colleague of a morning. Since she wasn't waiting down on Kyrkvgen, her colleague drove to her house and rang the doorbell, looked through the windows, then walked round the back and saw her. She was sitting on the floor, her face covered in knife-wounds, her neck almost sliced right through, her head lolling to one side and her mouth grotesquely open.

According to the preliminary report from the forensic post-mortem, there was evidence to suggest that her mouth had been arranged after death, even if it was theoretically possible that it had settled into that position of its own accord.

Rigor mortis starts in the heart and diaphragm, but is evident in the neck and jaw after two hours.

This late on a Friday evening the large foyer is almost deserted, aside from two police officers in dark-blue sweaters who are standing talking, and a tired-looking prosecutor emerging from one of the rooms dedicated to custody negotiations.

When Margot was appointed head of the preliminary investigation she was conscious of the pitfalls of being overambitious; she knew she had a tendency to be too eager, too willing to think on a grand scale.

Her colleagues would have laughed at her if she'd told them at the outset she was absolutely convinced they were dealing with a serial killer.

Over the course of the week Margot Silverman has watched the video of Maria Carlsson putting her tights on more than two hundred times. All the evidence suggests that she was murdered shortly after the recording was uploaded to YouTube.

Margot has tried to interpret the short film, but can't see anything special about it. It's not unusual for people to have a fetish about tights, but nothing about the murder indicates any inclination of that nature.

The film is simply a brief excerpt from an ordinary woman's life. She's single, has a good job, and has almost completed a course of evening cla.s.ses on drawing cartoons.

There's no way of knowing why the perpetrator was in her garden, whether it was pure chance or the result of a carefully planned operation, but in the minutes before the murder he captured her on film, so there has to be a reason for this.

Given that he's sent the link to the police, he must want to show them something.

The perpetrator wants to highlight something about this particular woman, or a certain type of woman. Perhaps it's about all women, the whole of society.

But to Margot's eyes there's nothing unusual about the woman's behaviour or appearance. She's simply concentrating on getting her tights to sit properly, frowning and pursing her lips.

Margot has visited the house on Bredablicksvgen twice, but she's spent most of her time examining the forensic video of the crime scene before it was contaminated.

The perpetrator's film almost looks like a lovingly created work of art in comparison to the police's. The forensics team's minutely detailed recording of the evidence of the b.e.s.t.i.a.l attack is relentless. The dead woman is filmed from various angles as she sits with her legs stretched out on the floor, surrounded by dark blood. Her bra is in shreds, dangling from one shoulder, and one white breast is hanging down towards the bulge of her stomach. There's almost nothing left of her face, just a gaping mouth and red pulp.

Margot stops as if by chance beside the fruit bowl on the table by the sofas, looks over at the guard, who is talking on the phone, then turns her back on him. For a few seconds she watches the guard's reflection in the gla.s.s wall facing the large inner courtyard, before taking six apples from the bowl and putting them in her bag.

Six is too many, she knows that, but she can't stop herself taking them all. It's occurred to her that Jenny might like to make an apple pie that evening, with lots of b.u.t.ter, cinnamon and sugar to caramelise them.

Her thoughts are interrupted when her phone rings. She looks at the screen and sees a picture of Adam Youssef, a member of the investigating team.

'Are you still in the building?' Adam asks. 'Please tell me you're still here, because we've-'

'I'm sitting in the car on Klarastrandsvgen,' Margot lies. 'What did you want to tell me?'

'He's uploaded a new film.'

She feels her stomach clench, and puts one hand under the heavy bulge.

'A new film,' she repeats.

'Are you coming back?'

'I'll stop and turn round,' she says, and begins to retrace her steps. 'Make sure we get a decent copy of the recording.'

Margot could have carried on out through the doors and gone home, leaving the case in Adam's hands. It would only take one phone call to arrange a full year of paid maternity leave. Perhaps that's what she would have done if she'd known how violent her first case would turn out to be.

The future lies in shadow, but the planets are approaching dangerous alignments. Right now her fate is floating like a razor blade on still waters.

The light in the lift makes her face look older. The thick dark line of kohl round her eyes is almost gone. As she leans her head back she understands what her colleagues mean when they say she looks like her father, former District Commissioner Ernest Silverman.

The lift stops at the eighth floor and she walks along the empty corridor as fast as her bulging stomach will allow. She and Adam moved into Joona Linna's old room the same week the police held a memorial service for him. Margot never knew Joona personally, and had no problem taking over his office.

'You've got a fast car,' Adam says as she walks in, then smiles, showing his sharp teeth.

'Pretty fast,' Margot replies.

Adam Youssef is twenty-eight years old, but his face is round like a teenager's. His hair is long and his short-sleeved shirt is hanging outside his trousers. He comes from an a.s.syrian family, grew up in Sdertlje and used to play football in the first division north.

'How long has the film been up on YouTube?' she asks.

'Three minutes,' Adam says. 'He's there now. Standing outside the window and-'

'We don't know that, but-'

'I think he is,' he interrupts. 'I think he is, he almost has to be.'

Margot puts her heavy bag on the floor, sits down on her chair and calls Forensics.

'Hi, Margot here. Have you downloaded a copy?' she asks, sounding stressed. 'Listen, I need a location or a name try to identify either the location or the woman ... All the resources you've got, you can have five minutes, do whatever the h.e.l.l you like, just give me something and I promise I'll let you go so you can enjoy your Friday evening.'

She puts the phone down and opens the lid of the pizza box on Adam's desk.

'Are you done with this?' she asks.

There's a ping as an email arrives and Margot quickly stuffs a piece of pizza crust in her mouth. An impatient worry line deepens on her forehead. She clicks on the video file and maximises the image on screen, pushes her plait over her shoulder, hits play and rolls her chair back so Adam can see.

The first shot is an illuminated window wavering in the darkness. The camera moves slowly closer, leaves brushing the lens.

Margot feels the hairs on her arms stand up.

A woman is standing in the well-lit room in front of a television, eating ice cream from the tub. She's tugged her jogging pants down and is balancing on one foot to pull her sock off.

She glances at the television and smiles at something, then licks the spoon.

The only sound in the room in Police Headquarters comes from the fan in the computer.

Just give me one detail to go on, Margot thinks as she looks at the woman's face, the fine features of her eyes, cheeks and the curve of her head. Her body seems to be steaming with residual heat. She's just been for a run. The elastic of her underwear is loose after too many washes, and her bra is clearly visible through her sweat-stained vest.

Margot leans closer to the screen, her stomach pressing against her thighs, and her heavy plait falls forward over her shoulder again.

'One minute to go,' Adam says.

The woman puts the tub of ice cream on the coffee table and leaves the room, her jogging pants still dangling from one foot.

The camera follows her, moves sideways past a narrow terrace door until it reaches the bedroom window, where the light goes on and the woman comes into view. She tramples the jogging pants off and kicks them towards an armchair with a red cushion. The trousers fly through the air, hit the wall behind the chair and fall to the floor.

2.

The camera glides slowly through the last of the dark garden and stops right outside the window, swaying slightly as if it were floating on water.

'She'd see him if she just looked up,' Margot whispers, feeling her heart beat faster in her chest.

The light from the room reaches beyond the leaves of a rosebush, casting a slight flare across the top of the lens.

Adam is sitting with his hand over his mouth.

The woman pulls her vest off, tosses it onto the chair, then stands for a moment in her washed-out underwear and stained bra, looking over at the mobile phone charging on the bedside table beside a gla.s.s of water. Her thighs are tense and pumped with blood after her run, and the top of the jogging pants has left a red line across her stomach.

There are no tattoos or visible scars on her body, just faint white stretch-marks from a pregnancy.

The room looks like millions of other bedrooms. There's nothing worth even trying to trace.

The camera trembles, then pulls back.

The woman takes the gla.s.s of water from the bedside table and puts it to her mouth, then the film ends abruptly.

'b.l.o.o.d.y h.e.l.l, b.l.o.o.d.y h.e.l.l,' Margot repeats irritably. 'Nothing, not a sodding thing.'

'Let's watch it again,' Adam says quickly.

'We can watch it a thousand times,' Margot says, rolling her chair further back. 'Go on, what the h.e.l.l, go ahead, but it's not going to give us a f.u.c.king thing.'

'I can see a lot of things, I can see-'

'You can see a detached house, twentieth-century, some fruit trees, roses, triple-glazed windows, a forty-two-inch television, Ben & Jerry's ice cream,' she says, gesturing towards the computer.

It hasn't struck her before, the way we're so similar to each other. Seen through a window, a broad spectrum of Swedes conform to the same pattern, to the point of being interchangeable. From the outside we appear to live exactly the same way, we look the same, do the same things, own the same objects.

'This is totally f.u.c.ked up,' Adam says angrily. 'Why is he posting these films? What the h.e.l.l does he want?'