‘Maybe she was talking to herself,’ replies Oliver.
‘No. There was another voice.’
‘Then she must have had another phone.’
My mind trips over the possibilities. Where did she get a second mobile? Why change phones?
‘Could the data be wrong?’ asks Ruiz.
Oliver bristles at the suggestion. ‘Computers in my experience are more reliable than people.’ His fingers stroke the top of the monitor as if worried that its feelings might have been hurt.
‘Explain to me again how the system works,’ I ask.
The question seems to please him.
‘A mobile phone is basically a sophisticated radio, not much different to a walkie-talkie, but while a walkie-talkie can transmit perhaps a mile and a CB radio about five miles, the range of a mobile phone is huge because it can hop between transmission towers without losing the signal.’
He holds out his hand. ‘Show me your phone.’
I hand it to him.
‘Every mobile handset identifies itself in two ways. The Mobile Identification Number (MIN) is assigned by the service provider and is similar to a landline with a three-digit area code and a seven-digit phone number. The Electronic Serial Number (ESN) is a 32-bit binary number assigned by the manufacturer and can never be changed.
‘When you receive a call on your mobile, the message travels through the telephone network until it reaches a base station close to your phone.’
‘A base station?’
‘A phone tower. You might have seen them on top of buildings or mountains. The tower sends out radio waves that are detected by your handset. It also assigns a channel so you’re not suddenly on a party line.’
Oliver’s fingers are still tapping at keys. ‘Every call that is placed or received leaves a digital record. It’s like a trail of breadcrumbs.’
He points to a flashing red triangle on the screen.
‘According to the call log, the last time Mrs Wheeler’s mobile received a call was at 12.26 on Friday afternoon. The call was routed through a tower in Upper Bristol Road. It’s on the Albion Buildings.’
‘That’s less than a mile from her house,’ I say.
‘Most likely the closest tower.’
Ruiz is peering over his shoulder. ‘Can we see who called her?’
‘Another mobile.’
‘Who owns it?’
‘You need a warrant for that sort of information.’
‘I won’t tell,’ replies Ruiz, sounding like a schoolboy about to sneak a kiss behind the bike shed.
‘When did the call end?’ I ask.
Oliver turns back to the screen and calls up a new map, covered in numbers. ‘That’s interesting. The signal strength started to change. She must have been moving.’
‘How do you know?’
‘These red triangles are the locations of mobile phone towers.
In built up areas they’re usually about two miles apart, but in the country there can be twenty miles between them.
‘As you move further away from one tower the signal strength diminishes. The next base station- the tower you’re moving towardsnotices the signal strengthening. The two base stations coordinate and switch your call to the new tower. It happens so quickly we rarely notice it.’
‘So Christine Wheeler was still talking on her mobile when she left her house?’
‘Looks like it.’
‘Can you tell where she went?’
‘Given enough time. Breadcrumbs, remember? It might take a few days.’
Ruiz has suddenly become interested in the technology, pulling up a chair and staring at the screen.
‘There are three missing hours. Perhaps we can find out where Christine Wheeler went.’
‘As long as she kept the phone with her,’ replies Oliver. ‘Whenever a mobile is turned on it transmits a signal, a “ping”, looking for base stations within range. It may find more than one but will latch onto the strongest signal. The “ping” is actually a very short message lasting less than a quarter of a second, but it contains the MIN and ESN of the handset: the digital fingerprint. The base station stores the information.’
‘So you can track any mobile,’ I say.
‘As long as it’s turned on.’
‘How close can you get? Can you pinpoint the exact location?’
‘No. It’s not like a GPS. The nearest tower could be miles away. Sometimes it’s possible to triangulate the signal from three or more towers and get a better fix.’
‘How accurate?’
‘Down to a street: certainly not a building.’ He chuckles at my incredulity. ‘It’s not something your friendly service provider likes to advertise.’
‘And neither do the police,’ adds Ruiz, who has started taking notes, boxing off details with doodled circles.
We know Christine Wheeler finished up at the Clifton Suspension Bridge on Friday afternoon. At some point she stopped using her mobile and picked up another. When did it happen and why?
Oliver pushes his chair away from the desk and rolls across the room to a second computer. His fingers flick at the keyboard.
‘I’m searching the base stations in the area. If we work backwards from five o’clock, we may find Mrs Wheeler’s mobile.’
He points to the screen. ‘There are three base stations nearby. The closest is on Sion Hill, at the bottom of Queen Victoria Avenue. The tower is on the roof of the Princes’ Building. The next closest is two hundred yards away on the roof of Clifton Library.’
He types Christine Wheeler’s number into the search engine. The screen refreshes.
‘There!’ He points to a triangle on the screen. ‘She was in the area at 3.20 p.m.’
‘Talking to the same caller?’
‘It appears so. The call ends at 3.26.’
Ruiz and I look at each other. ‘How did she get another mobile?’ he asks.
‘Either someone gave it to her or she had it with her. Darcy didn’t mention a second phone.’
Oliver is listening in. He’s slowly being drawn into the search. ‘Why are you so interested in this woman?’
‘She jumped off the Clifton Suspension Bridge.’
He exhales slowly, making his face look even more skull-like.
‘There must be some way of tracing the conversation on the bridge,’ says Ruiz.
‘Not without a number,’ replies Oliver. ‘There were eight thousand calls going through the nearest base stations every fifteen minutes. Unless we can narrow the search down…’
‘What about duration? Christine Wheeler was perched on the edge of the bridge for an hour. She was on the phone the whole time.’
‘Calls aren’t logged by length,’ he explains. ‘It could take me days to separate them.’
I have another idea. ‘How many of the calls ended precisely at 5.07 p.m.?’
‘Why?’
‘That’s when she jumped.’
Oliver turns back to the keyboard, typing in parameters for a new search. The screen becomes a stream of numbers that flash by so quickly they blur into a waterfall of black and white.
‘That’s amazing,’ he says, pointing to the screen. ‘There’s a call that ended at precisely 5.07 p.m. It lasted more than ninety minutes.’
His fingers are tracing the details when they suddenly stop.
‘What’s wrong?’ I ask.
‘That’s strange,’ he replies. ‘Mrs Wheeler was talking to another mobile which was routed through the same base station.’
‘Which means what?’
‘It means whoever was taking to her was either on the bridge or looking at it.’
13
There are girls playing hockey on the field. Blue-pleated skirts swirl and dip against muddy knees, pigtails bounce and sticks clack together. The word budding comes to mind. I have always liked how it sounds. It reminds me of my childhood and the girls I wanted to fuck.
The sports mistress is refereeing, her voice as shrill as a whistle. She yells at them not to bunch up and to pass and to run.
‘Do keep up, Alice. Get involved.’
I know of some of the girls’ names. Louise has the long brown hair, Shelly the sunshine smile and poor Alice hasn’t hit the ball once since the game began.