Joona thanks her and sits down on her chair, adjusts the media-player to full-screen and starts the clip. Just as Margot has described, it shows a thirty-year-old woman filmed in secret through her bedroom window as she pulls on a pair of black tights.

He sees her face, completely unaware, her downturned eyes, the calm set of her mouth, which then switches to something approaching lethargy. Her hair is hanging round her face, it looks like it's just been washed. She's wearing a black bra and she's trying to get her tights to sit properly.

There's a lamp with a clouded white shade and alabaster base in the window, and her shadow moves across the chest of drawers and the flowery wallpaper. She slips her hand between her thighs and tries to pull the thin nylon material up towards her crotch, and he can see her breathing through her mouth as the film ends.

'Did you see what you were looking for?' Adam asks, leaning over Joona's shoulder.

Joona remains seated in front of the screen, then plays the film again, watches her struggle with her tights, then freezes the picture after thirty-five seconds and clicks to advance it frame by frame.

'We've done that too,' Adam says, stifling a belch.

Joona moves closer to the screen and watches Maria Carlsson as she moves very slowly, breathing with her mouth open. Her eyes blink and her long lashes cast shadows across her cheeks. Her right hand sinks weightlessly between her thighs to her crotch.

'This won't do,' Adam says to Margot. 'We need to get on.'

'Give him a chance,' she replies.

Maria Carlsson turns jerkily towards the camera, the grey shadow crosses her face, as if she were being lifted up from a bath full of lead. Her lips part, the light from the lamp in the window shines on her face, making her eyes glow, and there's a shimmer in her mouth, then the film ends.

Behind Joona, Adam and Margot have started to talk about investigating the people in the drawing cla.s.s that Maria was about to set out for; they've already tried to find out if any of their names begin with 'H', but without success so far.

Joona moves the cursor and plays the last five seconds again. The light plays across her hair, her ear and cheeks, making her eyes shine, and then her mouth flashes.

He enlarges the image as far as he can without losing too much focus, then shifts the enlarged area so that it covers her mouth, and looks at the last few frames again. Her parted lips fill the screen, light shines in and the pink tip of her tongue becomes visible. He clicks to advance the image, frame by frame. The curve of her tongue comes into view, becomes lighter, and in the next shot it looks like a white sun fills the whole of her mouth. The sun contracts. And in the penultimate frame the glow has shrunk to a white dot on a grey pea.

'He took the jewellery,' Joona says quietly.

The two detectives fall silent and turn to look at him and the computer screen. It takes a few moments for them to interpret the enlarged image, the pink tongue and hazy stud.

'OK, we missed the fact that her tongue was pierced,' Adam says in a rasping voice.

Margot is standing with her legs apart and her hands round her stomach, and looks at Joona as he leans against the desk and gets up from the chair.

'You saw that she had a hole in her tongue and wanted to watch the film to see if the stud was there,' she says, picking up her phone.

'I just thought her mouth was important,' Joona says. 'Her jaw was broken, and she had her own saliva on her hand.'

'Impressive,' Margot says. 'I'll request an enlargement from Forensics at once.'

Joona stands still, staring at the pictures and maps on the wall as Margot speaks into the phone.

'We're collaborating with the BKA,' Margot explains once she's hung up. 'The Germans are way out in front when it comes to this sort of thing, in all forms of image enhancement ... Have you met Stefan Ott? Handsome guy, curly hair. He's developed his own programs, which J-lab ...'

'OK, so we've got an item of jewellery on the film,' Adam says, thinking out loud. 'The degree of violence is aggressive, fuelled by hatred ... probably jealousy, and ...'

Margot's inbox bleeps and she opens the email and clicks on the image so that it fills the whole screen.

In order to improve the contrast of the stud itself, the image enhancement software has changed all the colours. Maria Carlsson's tongue and cheeks are blue, almost like gla.s.s, but at the same time the stud is clearly visible.

'Saturn,' Margot whispers.

At the end of the stud piercing Maria's tongue is a silver sphere with a ring around its equator, just like the planet Saturn.

'That's not an "h",' Joona says.

They turn and see that he's looking at the photograph of Maria's Filofax where it says 'cla.s.s 19.00 squared paper, pencils, ink', then on the line below the letter 'h'.

'That's the symbol for Saturn,' he says. 'It actually represents a scythe or sickle. That's why it's slightly crooked, and sometimes it's crossed up at the top.'

'Saturn ... the planet. The Roman G.o.d,' Margot says.

31.

Joona and Margot have taken their shoes off and are standing looking through a pane of gla.s.s. The room inside is warm and damp.

'I've tested for allergens, and it turns out that I'm allergic to mindfulness,' she says.

To the strains of Indian music, about thirty perspiring women are moving with mechanical symmetry on their yoga mats.

Margot got five officers to check through Maria Carlsson's Internet traffic once more: her email, Facebook and Instagram accounts. The stud in her tongue is only visible in a few pictures, and is only mentioned by one of her friends on Facebook before all communication between them ceased.

'You got lick it, before we kick it. Me too wanna pierce my tongue.'

The woman who had posted that was called Linda Bergman, and she was an instructor in Bikram yoga in the centre of Stockholm. They were in very regular contact for six months before she suddenly unfriended Maria.

Linda Bergman emerges from the staffroom dressed in jeans and a grey sweater. She's suntanned, and has quickly showered and put on some make-up.

'Linda? I'm Margot Silverman,' Margot says, shaking the woman's hand.

'You didn't say what this was about, and I can honestly say that I have absolutely no idea,' she says.

They walk along the pavement in the direction of Norra Bantorget while Margot tries to get Linda to relax by asking about Bikram yoga.

'It's a form of Hatha yoga, but takes place in a room with high humidity, at a temperature of forty degrees,' Linda explains.

They enter the former playground in front of the old Norra Latin School. The spherical fountain shimmers silvery white, and the wind keeps scattering showers of tiny droplets.

'The founder's name is Bikram Choudhury ... he created a series of twenty-six positions that are actually the best I've ever tried,' she goes on.

'Let's sit down,' Margot says, patting her stomach.

They sit down on an empty park bench beside the fence facing Olof Palmes gata.

'You used to be friends with Maria Carlsson on Facebook,' Joona says, drawing a deep vertical line in the path, raising a little cloud of dust.

'What's happened?' she asks warily.

'Why did you unfriend her?'

'Because we no longer have anything to do with each other.'

'But you seem to have been in very close contact for several months,' Margot says.

'She came to a few cla.s.ses, and we started talking, and ...'

Linda tails off and her gaze flits anxiously from Margot to Joona.

'What did you talk about?' Margot asks.

'Can I ask if I'm suspected of having done something?'

'You're not,' Joona says.

'You knew that Maria had a piercing, that she had a tongue-stud?' Margot goes on.

'Yes,' Linda says, and gives a slightly embarra.s.sed smile.

'Did she have several different studs?'

'No.'

'Do you remember what hers looked like?'

'Yes.'

Linda stares at the old school building and the play of the shadows under the trees for a moment before replying: 'It had a tiny model of Saturn at the top.'

'A tiny model of Saturn,' Margot repeats, very gently. 'What does that mean?'

'I don't know,' Linda says blankly.

'Is it to do with astrology?'

Linda looks over at the trees again, and kicks the ground with her trainers.

'Do you know where she got it from? It doesn't seem to be for sale anywhere, not from any of the usual Internet sites, anyway.'

'I don't understand where this is going,' Linda says. 'I've got another cla.s.s soon, and-'

'Maria Carlsson's dead,' Margot interrupts, with quiet seriousness. 'She was murdered last week.'

'Murdered? She was murdered?'

'Yes, she was found on-'

'Why are you telling me?' Linda interrupts and stands up.

'Please, sit down,' Margot says.

'Maria's dead?'

Linda sits down, her eyes drift off towards the fountain, and she starts to cry.

'But I ... I ...'

She shakes her head and hides her face in her hands.

'Did you give her the stud?' Joona asks.

'Why the h.e.l.l do you keep going on about that tongue-stud?' she snaps. 'Find the killer instead. This is completely sick!'

'Did you give her the stud?' Joona repeats, drawing a short line across the first one.

'No, I didn't,' she replies, wiping the tears from her cheeks. 'She got it from a guy.'

'Do you know the name of the guy?' Margot asks.

'I don't want to get involved,' she whispers.

'We respect that,' Margot nods.

Linda looks at her with bloodshot eyes, and purses her lips.

'His name is Filip Cronstedt,' she says quietly.

'Do you know where he lives?'

'No.'

'Was Maria going out with him?'

Linda doesn't answer, just stares down at the ground as the tears begin to fall again. Joona adds the last curve to the symbol with his stick and leans back.

'Why did she have a model of Saturn on her tongue-stud?' Margot asks carefully. 'What does it mean?'

'I don't know. Because it looked nice,' she says weakly.

'In Maria's diary there's a symbol written in ten different places it's the old symbol for Saturn,' Margot says, and points to the ground.