‘You don’t talk to the press, OK? You just never do it. We have people to do that for us. The media liaison officers – leave it to them, all right? Because as soon as you start talking to journalists, as soon as you start mouthing off with your little theories, then every mummy’s boy with an iMac and a grudge against society starts crawling out of the woodwork, boasting on all the social network sites about how antisocial they’ve been. And once they do – once they start claiming to be Bob the Butcher – then we have to follow it up, chase down their IP addresses, go round to their house and tell them they’ve been very, very naughty. So we don’t talk to the press, the MLOs do. OK?’

‘Fair enough,’ I said.

He saw that I wasn’t going to argue with him, and he softened.

‘Especially to this Scarlet Bush,’ he continued. ‘She’s poison. They say she’s got a tabloid brain and a broadsheet mouth. Or is it the other way round?’

I hesitated. ‘But you’re taking Bob the Butcher seriously?’

‘We have no choice,’ Gane said. ‘If someone claims to have committed murder, then we have to take them seriously. And run them down. It’s sort of our job.’

Mallory said, ‘You’ve got the IP address for this Bob the Butcher?’

‘Not yet, sir,’ Gane replied. ‘He’s running his messages through some kind of anonymiser or anonymous proxy. Probably Tor or 12P. It’s an intermediary that’s meant to act as a wall between the user and the rest of the digital world. Most of the child pornography online is run through anonymisers. It’s the deep web, designed to bury an IP address. Bob’s been in touch again, if anybody’s interested.’

Gane hit a couple of keys. On the big TV screen a social network site replaced the photograph of the seven boys. A black-and-white photograph the size of a postage stamp featuring a thin-faced man in a suit, tie and hat, an insolent cigarette dangling from one corner of his mouth. A face from history.

Mallory read the message aloud: ‘I am become Death, the destroyer of worlds. Behold the dark angel of the people, righteous avenger of the dispossessed. Bob the Butcher is coming. Kill all bankers. Kill all pigs. #killallpigs.’

Gane laughed. ‘Now there’s someone who has played one too many computer games.’

Mallory wasn’t laughing. ‘The first part is from Robert Oppenheimer, the father of the atomic bomb,’ he said. ‘“I am become Death, the destroyer of worlds”. The photograph is of Oppenheimer. He was quoting a Hindu scripture, the Bhagavad-Gita. He said it after they tested the first nuclear weapon. After the world changed for ever.’

‘I’ll find him,’ Gane said, stabbing some keys.

The photograph of the seven boys returned to the screen.

‘The school is the key,’ Mallory said. ‘Potter’s Field.’