‘Are you all right?’ Cullen asks, almost genuinely concerned.

She bites her lip again. Oh, now Cullen sees it. She is doing her best to keep herself together. He supposes that the maths helps somehow.

But first he needs to find out what else she knows.

‘Let’s not get ahead of ourselves. To go back to Satnam, you have no ideas about what led up to this? What made her say she wanted to leave? Family problems? Workload? Boyfriend? Nothing on her phone?’ He holds his breath.

She shrugs. ‘She had a boyfriend, Luke, but she broke up with him months ago. Since then she’s just been focusing on her work. She goes to the library a lot. Except…’ She tails off. A bead of sweat rises up on Cullen’s forehead. He wipes it away.

‘Except what?’

‘I’m there 24.66 hours a week on average and I haven’t seen her there.’

‘I see.’ He doesn’t like the direction the conversation is taking. He wants Nevis to tell him what she knows without raising questions of her own. ‘As you might know, Satnam took a catch-up class with me last summer. I’ve been seeing her on and off since then to make sure she stays caught up. I can attest to her increased output. She’d been working hard.’

‘Oh, OK.’ Nevis looks unconvinced.

‘But if we had her phone?’ he goes on, with the most casual tone he can muster.

‘It’s passcode protected.’

‘Do you know where it is? There was a suggestion it had been lost.’

‘Oh.’ She compresses her lips and falls silent.

He changes tack again. ‘You’ve probably also heard that Satnam’s parents are saying what happened was an accident?’

‘Yes,’ she says.

‘And what do you think about that?’

‘Probability close to zero.’

‘You are aware, I suppose, that an amount of Ritalin was found in Satnam’s blood? A great deal of alcohol too.’

She nods.

‘From what I understand, a combination of that particular drug when taken with alcohol can lead to all kinds of mental confusion, hallucinations even. Satnam might have thought she was going somewhere quite different from the bridge.’

He watches her make a mental calculation and then the lines on her face appearing. ‘If it was an accident why would she have taken so many pills? And she doesn’t usually drink, or not much anyway.’