He brought the rifle around, looking for the second ambulance.

Superintendent Enfield’s direct line rang and he picked it up. It was Wendy Aspden, who asked him to hold, then put Willoughby-Brown through. ‘Superintendent, it’s all turning very nasty at the stadium,’ he said.

‘Yes, I’m just talking to my inspector. Five wounded and our helicopter has just crashed.’

‘Superintendent, I’m told you have undercover armed officers on duty at the stadium.’

‘Several. Including an undercover SAS unit.’

‘You need to tell them to take out the ambulances. We believe they are bombs being driven by jihadists. They need to move now.’

‘What? Are you sure?’

‘Superintendent, you can either be a hero today or you’ll take the blame for one of the biggest terrorist atrocities this country has ever seen. The gates have been opened to allow the ambulances in but they are Islamic State terrorists. Get your men over to the ambulances now and tell them to do what has to be done.’ Willoughby-Brown ended the call.

Shepherd focused his scope on the second ambulance, which had just come to a halt towards the middle of the stadium. The man’s face centred on the scope. It was another of the men given a Syrian passport by Yusuf. The man was holding his right hand up and Shepherd could clearly see the trigger. He went to chamber a round but his magazine was empty. He yanked it out, grabbed a full magazine by the cushion and slammed it in. He chambered a round, his heart racing. He resisted the urge to rush his shot. He aimed, he controlled his breathing, and he slowly squeezed the trigger. The windscreen exploded. He chambered a second round and fired again, catching the man in the throat and severing his spine. The head lolled to the side and the hand holding the trigger dropped. Shepherd held his breath but there was no explosion.

He took his eye away from the scope. Almost unbelievably, the bout was continuing. The two boxers were exchanging punches and the crowd were roaring, many of them on their feet.

Another ambulance was moving slowly in his direction but as he watched the stadium roof blocked it from view. Another had come to a halt close to a podium from which two TV cameras were broadcasting the fight.

‘There are armed cops down there,’ said Willoughby-Brown.

‘They’d better move quickly,’ said Shepherd.

He put his eye to the scope, focused on the remaining ambulance he could see and immediately recognised the driver. He aimed at the head, held his breath and squeezed the trigger. The window exploded and he ejected the cartridge and rammed home another. Aim. Breath. Squeeze. The head imploded and jerked back. Shepherd looked at Willoughby-Brown. ‘I’ve done all I can,’ he said. ‘It’s up to the cops now.’