The tool also showed where people were reading a story. An article about Montevideo might see a surge of interest from IP addresses in Uruguay. A story about bullfighting would light up in Madrid.

For the three articles just published, Toby had built an extra gadget that captured a list of IP addresses as they clicked onto the articles.

‘That way,’ Casey explained to Dash, ‘we can see everyone who is interested in these precise three topics. Whoever it is might well be particularly vigilant about coverage if they are the sort of people who actively don’t want publicity.’

‘Bait over a trap,’ said Dash. ‘Electronically. But the IP addresses might not give you enough.’

The IP addresses – Internet Protocol addresses – could be linked back to a geographical area, but they might also mislead.

‘If we can find one IP address that’s interested in all three of these articles,’ explained Casey, ‘Toby will be able to make a pretty good guess at where that person is right now.’

‘OK,’ Dash shrugged. ‘But how can you be sure the person will know the articles have been published?’

‘Hopefully, they’ve got news alerts set up for their companies,’ said Casey. ‘And those alerts will be triggered by the Post publishing any reference to those companies.’

Dash read through the three articles. ‘Arachne Incorporated. Portunus Marine. And what’s the third?’

‘I added one letter to “aces”,’ said Casey. ‘And then started the next sentence, “Limited crowd enthusiasm . . .”’

Dash winced. ‘No wonder Arne wasn’t thrilled. Aceso Limited,’ Dash spelled it out. He laughed. ‘Honestly, Casey.’

‘It might work,’ she protested. ‘And we haven’t got many options.’

‘Don’t you think you might just end up with a whole load of addresses that read everything on the Post website?’ said Dash.

‘Toby’s analysing a few other Post articles,’ said Casey. ‘So we can eliminate the bots.’

‘Might work.’ Dash turned to her. ‘And then what?’

‘And then . . .’ Casey stared across the newsroom. ‘Then I don’t know.’

It was very late when Toby came into the investigations room. Hessa and Miranda were working. Casey was curled up on the tatty sofa, half-asleep. Emily and Dominic were prowling round her mind; Zohra screaming in the dark.

‘I think I’ve found it,’ Toby said, and Casey sat up abruptly, rubbing her eyes.

‘Show me?’ she asked.

Toby kneeled down on the floor beside her with his laptop.

‘There are about twenty addresses that have read all three articles,’ he said. ‘Addresses that aren’t routine Post readers or from some automated system.’

‘And then you . . .?’

‘Cross-referenced the ownership against that list of companies named after Greek deities.’

‘And?’

Toby turned the laptop towards her. A huge house filled the screen. Jacobean, Casey guessed. Rows of stone-mullioned windows gazed out over rolling parkland and an exquisite lake.

‘Christ,’ said Casey.