We walk slowly towards the edge of town. It’s late and there are fewer people around. A couple kiss languidly in a doorway. A bunch of kids run across the street. Duval stops, his eyes passing fleetingly from one to the other. I watch his face, suddenly overcome with sadness for him. ‘You’re still looking for him, aren’t you?’ The boys disappear round the corner and Duval stares after them down the empty street.

‘I must have looked into the face of every child in Nogales and wondered whether it’s him.’

‘But he’d be older, right? Eighteen or something?’

‘Twenty-one ... if he made it that far. If the street has been his life he might be consigned to Reuben’s fate, living some totally predatory existence in the shadows, ending up face down in a ditch because he can’t pull together the five bucks to pay off a scam.’

I want to offer some comfort but there is nothing to say that doesn’t sound completely fatuous. ‘Maybe he’s been brought up by a Mexican family, gone to a good school.’

‘He’s almost certainly dead,’ Duval replies bluntly. ‘And maybe that was the best escape for him.’

He sees my expression. ‘Alice, I tried everything. Social services, Border Patrol, the Mexican authorities—’ He breaks off. ‘I got the feeling they could have helped but they weren’t that interested. For a while I hung around with the tunnel kids in the hopes of the older ones remembering something but . . .’ He shakes his head. ‘I had very little to go on.’

I’ve read about the tunnel kids. The two Nogales might be separated by the wall, but they are forcibly linked by a sewage system, a great long metal pipe that snakes underneath the two cities. It’s a route to the other side for pretty much everyone, coyotes and smugglers alike, but it’s also home to the vagrant kids of the town who consider it their patch and exact a pretty heavy toll from anybody trying to pass.

‘And nothing?’

‘I followed a few breadcrumbs. Got hopeful once or twice, but nothing led anywhere. It was always going to be impossible.’ He shrugs. ‘It’s not like they put missing Mexican kids on the back of milk cartons.’

‘I’m sorry. Really.’

‘Ironic, isn’t it?’ He smiles but it doesn’t reach his eyes. ‘It’s the mundanity of life everyone tries to escape from but it was the ordinary stuff I wanted for him. The-sit-at-a-table-and-do-homework stuff.’

I think of Jack and Emmy with Benjamín, how I wouldn’t be there when they came home from school. I think how easily I’d deserted them at the merest whiff of adventure.

‘And this El Turrón? What would you do if you ever found him?’

Duval’s face is impassive. ‘Kill him.’