And then Swire. ‘That’s not him,’ she repeated. ‘Stop talking, Wolfe.’ A note of steel now. ‘You had one task. It is concluded. No further action necessary. We’re standing down all units. Negative. Cancel. Thank you everyone.’

The crowd slowed outside the station as it merged with the flow of commuters coming over from King’s Cross. I figured that I had one minute to stop him before he disappeared inside the station. Once on a mainline train or down on the tube or on the concourse of the station itself, the man with the red backpack would simply touch his hands together and the world would blow apart.

The battery he probably already held in one hand would create an electric current connecting it to a simple terminal held in the other. The current would then pass down two wires and into that red backpack – a discreet slit would have been cut in the side – where a modified light bulb would trip a detonator stored inside a small tube. This would trigger the main charge – the hydrogen peroxide I had watched him buy with eleven £50 notes on CCTV.

At the same time he had bought a bulk supply of six-inch steel nails. Sacks of them. They would be taped to the outside of the main charge to inflict enough misery to last for several hundred lifetimes.

If it detonated.

If he was that smart.

If he hadn’t messed up the cook.

I choked down a lump of hot bitter nausea as it rose in my throat.

‘You’re wrong,’ I said. ‘That’s him. Contact.’

I had been inside his lock-up. I had seen the hundreds of empty bottles of hair bleach. I had watched the CCTV footage of the day he bought them until my eyes were burning with the sight of him.

I didn’t need the photographs taped to the dash. I knew him. He was in my head.

He could not hide from me.

‘Stand down all units,’ a self-consciously calm voice was saying. ‘Do you copy me, Delta 1?’

‘No,’ I said. ‘You’re breaking up.’

Thirty seconds now.

And among all those crowds, and surrounded by all those firearms, I was on my own with the man who was planning to die.

I once attended a lecture at the police academy in Bramshill, Hampshire – the Oxford and Cambridge of police higher education.

An FBI agent had been flown over to help us combat terrorism. I had been impressed by the whiteness of the agent’s teeth. They were a fine set of teeth. Very American teeth. But what impressed me more was that the man knew his business.

His teeth shining, he told us that the FBI had identified twenty-five threat areas for terrorist activity. It wasn’t quite an A to Z, but it was close – it was an A to T, from airports to tattoo parlours.

Everywhere, basically.

The Fed also suggested what possible terrorists might look like.