Scream: A DCI Mark Lapslie Investigation (6 page)

She smiled at Lapslie’s blank expression. ‘It’s okay. Look, any sound that’s been recorded and then translated into digital format exists as a series of numbers. The numbers represent the characteristics of the sound, like volume and frequency – taken
a number of times per second. If you think of the original sound as a smooth curve then the digital representation is a series of steps which try to match the curve. The more steps you have per second, the smoother the resulting profile looks. So, 44,100 samples per second, with 16 bits per channel, and two channels, will result in a bit-rate of 1,411,200 bits per second. That’s almost perfect sound quality. A sample recorded at a rate of, say, 320,000 bits per second is listenable. Even 190,000 bits per second is more than acceptable for most purposes. By the time you get down to 128,000 bits per second then you can hear a noticeable degradation in quality, something like a radio broadcast, and 64,000 bits per second is more like listening to something over a phone than anything else. Clear?’

‘I have no idea what you’re talking about,’ Lapslie said in a heartfelt voice.

She smiled. ‘Don’t worry. If you take one thing away from this, take away the fact that the sound file was recorded to preserve as much information from the source as possible, given the constraints of data storage.’ She paused, and fixed him with a piercing gaze. ‘Whoever recorded it wanted it to sound good. They
cared
about the sound quality.’

Lapslie thought for a moment. ‘You’ve indicated that there’s a trade-off at work there – sound quality versus size of file. Would it be possible to record it in a way that preserved as much of the sound as was technically possible?’

‘Oh yes,’ she said. ‘But that would take up a huge amount of hard disk space, assuming it was being stored on a computer.’

‘So why would this person compromise on the sound quality?’

‘So they could store more sound files,’ she said brightly. Her face fell. ‘Oh. That means—’

Lapslie finished the thought for her. ‘That means whoever recorded this sound file probably has so many files that they
have problems with the storage, so they have to compromise the sound quality.’

‘So whoever is doing the screaming—?’

‘—is either not the only screamer—’

‘—or they’ve done a lot more screaming than just this.’

The lab was silent for a while as the three of them absorbed the implications of what they had just worked out.

‘What else can you tell me?’ Lapslie asked quietly.

Sara was considerably less bubbly now than when Lapslie and Burrows had come in. ‘I ran the file through a series of filters to reduce hum and static and to enhance the audio frequencies. First, judging by the slight echo on the screams, she’s in a large and relatively open space.’

‘She?’ Lapslie asked.

‘It’s definitely a woman’s voice. Analysis of timbre confirms that. Whatever it is that’s happening to her to cause her pain happens twenty-seven times. That’s the number of cries there are. And thanks to the fact that it’s a stereo recording we know that either she was moving around while the recording was being made or the person that made the recording was moving around.’

‘Either way,’ Burrows interjected, ‘it suggests that whatever is going on is not straight torture, with the victim restrained and the torturer standing over them. This was mobile.’

‘The recording is original and unedited,’ Sara added. ‘What’s there is what actually happened, from the moment the “Record” button was pressed to the moment the “Stop” button was pressed.’

‘How do you know?’ Lapslie asked.

‘We use a specialised piece of software to detect signs that a file has been edited. It was originally developed for use by the KGB, but it’s available on the open market now. It analyses any
recording for signs of tampering, like changes in background noise, artefacts left by stopping and starting recording, and a whole load of other signatures that can be electronically detected.’ Her face darkened. ‘And I’m pretty sure that she dies at the end.’

‘Because …?’

‘Because of the very last sound.’ She turned to the computer and, using the mouse, highlighted a particular mountain peak at the end of the graphical representation. She pressed the ‘Play’ button on the screen.

Lapslie drew his breath in sharply as the final moments of the sound file played: a low, indeterminate noise that still managed to chill the spine, despite its ambiguity.

‘That could be anything,’ he said.

‘It’s a death rattle,’ she replied. ‘The last breath catching on mucal secretions in the back of the throat. I’ve—’ She stopped, swallowed – ‘I’ve heard it on recordings before.

He raised an eyebrow questioningly.

‘Usually last confessions from someone in hospital, recorded by a police witness,’ she added. ‘Not one of the more pleasant aspects of the job, but we have to be able to prove that they are genuine for the courts.’

‘“Life, like a child, laughs, shaking its rattle of death as it runs,”’ Lapslie murmured softly.

Burrows frowned. ‘What?’

‘Nothing. Just a phrase.’

‘Oh,’ Sara said after a few moments, ‘I almost forgot. The file was recorded in the UK, probably in Essex.’

‘How do you know?’

‘Electrical Network Analysis. ENA.’

Lapslie looked at her blankly.

‘If the recording device was connected to a mains supply then
there’s going to be a modulation of the audio signal due to electrical network frequency fundamentals, their harmonics and sub-harmonics. We use specialist forensic software to detect the number and relative amplitudes of electrical network frequency components, detect what type they are – mains, uninterruptible power supplies, etcetera – and compare them to a database of various national mains supplies that’s been compiled by acoustic forensic experts across the world. This allows us to identify the grid that generated the components – continental Europe, UK, USA and so on. We can also, if we’re lucky, narrow the components down to a particular generator or substation, which is what we did in this case.’

She sighed. ‘I wouldn’t want you to think that it’s easy. We have to ensure that our own equipment housings, equipment, cabling and so on is properly screened, that the equipment which we’ve connected together for recording and analysis purposes is connected in a way that avoids earth loops that would give rise to an induced local electrical network frequency and that safeguards are in place to avoid radio frequency interference from mobile phones which could bleed into the recording and analysis equipment.’

‘And that,’ Burrows said, ‘is why I keep asking for a larger and larger budget. Remember that, when you’re Chief Constable.’

‘I will, I will,’ he promised, deeply impressed by the technician’s grasp of her subject. He turned back to Sara. ‘Anything else?’

She shrugged. ‘I’m still analysing the file for background noises – anything extraneous to the woman’s actual voice. I’ll be able to tell you more in a day or two.’

Lapslie nodded. ‘Okay, I’ll be back then.’ He hesitated. ‘Good work,’ he added awkwardly.

On the way out, something occurred to him. He turned to
Burrows. ‘Wait,’ he said urgently, ‘I forgot to ask where the email was sent from.’

‘Different lab,’ Burrows said succinctly, and kept walking.

Back to the main spine corridor, down two spurs (literally, as the building appeared to be built on a hill and the spine corridor dropped two steps every few hundred yards) and through another security door, and Burrows led Lapslie into a lab that looked to all intents and purposes like the exact twin of the one he had just left. Even the girl sitting at the computer was the same age and build, except this one was a brunette rather than a blonde.

‘Lucy, this is DCI Lapslie. Mark, this is Lucy Whiteman.’

Lapslie nodded at her. ‘Pleased to meet you.’

‘That email with the attached sound file,’ Burrows continued. ‘Any joy on tracing the email address?’

She nodded. ‘The address is a dead end,’ she said. ‘It’s a Hotmail account, set up anonymously. There’s no way of tracing it back to a particular person.’

‘What about the location it was sent from?’ Lapslie asked.

‘Don’t let me get you bogged down in detail—’ she started.

‘Oh, please don’t,’ Lapslie said.

‘—but the IP address is one of the new Internet Protocol version 6 addresses. An IP address is a unique identifying number which identifies a particular computer. They can be reassigned, which means that a single IP address might, over time, refer to many different computers, but only one at a time. Not that many of the version 6 addresses have been assigned as yet. This one is part of a set that are dynamically assigned static IP addresses; what we refer to as sticky IP addresses. That means that if the computers that they point to are powered-on for extended periods of time, the address leases are usually set to long periods and simply renewed upon expiration.’

She leaned back and gazed at Lapslie expectantly.

‘Which means?’ he prompted.

‘Which means that there’s a good chance that the IP address from which this email originated is still in use by the same physical computer. If not, it’s likely to be one in the same locality.’

‘Okay,’ Lapslie said after a pause. ‘And that locality is …?’

‘Oh,’ she said. ‘Yes.’ She leaned forward and typed something into her keyboard. ‘It’s located in a hospital in Chelmsford. I’ve got the address here. It looks like it’s one of a set associated with an internet café based in the hospital.’

‘What’s the hospital’s name?’ Lapslie asked grimly.

‘Chelmsford General,’ she said.

Lapslie felt his stomach clench into a hard ball.

Burrows looked at Lapslie, detecting something in his stance. ‘Familiar to you?’

‘You could say that,’ Lapslie snapped. ‘I’m being treated there. I know it well.’

‘And someone there knows you well,’ Burrows said simply.

That thought echoed around Mark Lapslie’s mind as he walked down the hill from the forensics lab towards the main gate and the car park. And as he walked, concern began to bubble up within his chest like a dark, corrosive liquid. He’d not spoken to Charlotte since he’d got back. She’d texted him, but that might have been days ago. Or it might not have been her, but someone pretending to be her.

He knew he was panicking but he couldn’t help it. He was being targeted. The sound file had been intended for him, and sent by someone who knew him. There was no doubt about it. Knowing that the person who had sent the email obviously knew about him and details of his life, didn’t that make it more likely that the target was someone he knew? Such as his girlfriend? By the time he’d got to his car he could feel his pulse
racing, the blood pounding in his neck and his temples, and he was sweating. He pulled his mobile phone out and called Charlotte, but it went straight to her voicemail.

‘It’s Mark,’ he said. ‘I’m back. Can you call me?’ He knew his voice sounded shaky, but he couldn’t help himself. He needed to know she was safe.

He hardly saw the Essex countryside flashing past as he drove from the forensics lab to Chelmsford. Pulling into the hospital’s pay car park, he found himself staring up at the blank windows balefully, wondering which one of them hid the person who had sent him the email. If, of course, the person actually worked at the hospital or was a patient there. Perhaps they’d been visiting someone, or attending an outpatients’ clinic. Perhaps they had just slipped in to use the internet café and then left again. There was no way to know.

He headed straight for the office Charlotte shared with another doctor, but the door was locked. He debated sticking a note on it, but if she was okay, if she was still alive and not the subject of some serial killer who wanted to taunt Lapslie, then she would think he was getting desperate, and he didn’t want her to think that. He could come back later. He
would
come back later.

He glanced at his watch as he entered the hospital. He had twenty minutes before the synaesthesia workshop started at 6.30 – timed so people could get there after work. Enough time to take a look at the internet café.

It was located on the ground floor, just off the main hall and next to the real café. Twelve computer terminals were lined up along two benches, their cables carefully hidden away from fidgeting hands. Patients or visitors could access them for 50p per half hour, paid to a young man who sat behind a counter and presumably provided some level of tech support. Useful,
Lapslie assumed, for checking emails, keeping up to date with eBay bids or finding out what had happened on
EastEnders
the previous night, if they’d missed it because they were giving birth or something. The screens all faced into the central aisle, preventing people from obviously accessing pornography unless they were particularly shameless. Lapslie assumed that there were firewalls set up anyway, to block access to dubious sites. Tiny blue LEDs glowed in the centre of the plastic border around the screens. Half of the terminals were occupied: three by patients in dressing gowns and pyjamas, one by a doctor in a white coat and one by someone dressed casually in jeans and T-shirt.

Lapslie looked around, hoping to see a security camera pointing into the centre of the internet café, but there was nothing. No chance to access CCTV footage of his mysterious interlocutor.

He kept watching for a while, not sure what he was watching for, then abruptly turned and left. Resisting the urge to go back to Charlotte’s office, he headed up to the fourth floor.

The synaesthesia workshop was held every week, under the auspices of the Neurology Department. Approaching the disinfectant-tinged quiet of the central waiting area, he found that six chairs had been arranged in a half-circle around a seventh. Five of the six chairs in the half-circle were occupied by people who were chatting in desultory fashion or just looking around blankly, waiting for something to happen. Lapslie recognised four of them from the last workshop. The fifth – a man of about Lapslie’s age – was presumably a newcomer. The sixth chair was obviously set out for Lapslie and, as usual, he was last to arrive. The seventh chair, the one set at the focus of the other six, was also empty. That was where the man leading the workshop – Doctor Garland – would sit.