A small group stopped by the side of his truck just after midnight. Jews with white and blue Israeli flags, chanting something he didn’t understand.

He watched their hesitation as they stared down the street.

‘It’s quicker this way,’ said one of them, a man.

‘I think they’ve closed the south entrance,’ said one of the girls.

‘You have to wait in line for a bag search. You need to be getting home.’

‘But it’s important,’ said the girl.

He had had enough. The killer shouted down from the cab: ‘You guys want to get a ride right to the heart of this thing? Ever been in a police truck?’

They looked up. The driver was a bright-eyed cop. He was smoking too — a lit cigarette hung from his lips. This was not like some of the hard-faced cops they’d met; this guy seemed different.

‘Where to?’ one of them asked.

‘I’m going to the police compound.’

‘I’d like a lift,’ said one of the girls.

‘I got an empty truck here and I’m going through the crowds and down to the compound. You want to get a VIP seat to the vigil?’

This group felt invulnerable, in the middle of a huge police operation. In the middle of their people. Why would a group of five fear a single cop? It was all about gaining trust.

‘I’m going now, anyway, make your minds up,’ said the killer.

‘I think we should do it,’ said one of the guys. ‘We should take the time to do this.’

‘Okay, that’s final,’ said the killer. He jumped down from the cab, opened the back of the van and let them pile in, then he shut and locked the door. There was a light in the back to prevent them from getting scared, but the moment he started to drive he would switch it off. That way, they wouldn’t be able to see where the gas was coming from and wouldn’t be able to cover the vent.

‘Ready to go?’ he shouted from the front cab. He could hear their cheers as he started the engine, drove away from the curb and into the traffic.

He had read from the Nazi reports of the use of gas vans that screaming was a problem. They had tried to stop this distraction by gagging the victims prior to putting them in the vans, but that just wasn’t possible in this case. The killer couldn’t gag them, so he had another plan.

Inside the back of the van, the group lurched left, then right, but they were laughing as they fell across each other. After a minute of driving, the smell in the back of the truck was noticeable. They sniffed and stopped laughing, and someone started to cough.

The killer heard a bang on the side of the van. ‘Stop the truck,’ a man’s voice called. ‘We’ve got exhaust fumes in here.’

In the cab, the killer leaned forward and flicked off the light. In the back of the van, darkness fell.

As the killer turned up 2nd Avenue, he could hear them screaming. He knew it would take ten to fifteen minutes for the gas to kill them. He couldn’t risk others hearing them, so he switched on the sirens and continued to drive; the loud wailing of a police truck was all that he or anyone else could hear.

Union Square Park

March 14, 4.12 a.m.

The night had come and gone. No dead bodies, no explosions. The first lights of dawn were arriving and most of the crowd had dissipated. There were a few left in small huddles, wanting to ride it out until morning.

Harper had been forty-eight hours without sleep but was still standing, while Eddie sat down, his head low in the collar of his Puffa jacket. ‘When do we get to go home?’ said Eddie. ‘He’s not showing up. He’s probably tucked up in bed, laughing at us.’

‘We go when they all go,’ said Harper, as his phone rang.

Denise had called throughout the night, with sharp, disturbing updates. There was no tiredness in her voice. She had spent the night going over reports about Sturbe with Abby Goldenberg’s father and hunting any link between Lucy Steller and the killer.

‘What have you got?’ said Harper, his eyes still scanning a park now scattered with litter.

‘Get this. Sturbe had an 88 tattooed on his chest. I have no doubt that our killer would copy this. And about future kills, he was involved in the destruction of the Great Synagogue.’

‘Meaning?’

‘I think he could try to destroy something big. He’s copying this guy’s ghetto history, and if he is, he’s got a lot more kills to get through. Sturbe’s own notes indicated he killed over forty people.’

‘Anything you got that might help us track him down?’

‘We found something on the Lucy front. We’re close. We think there might have been a boyfriend.’

‘A boyfriend?’ Harper struggled with the thought.

‘Yeah, her friends said she went out with a slightly older guy. I asked if it was Heming, they looked at the pictures in the papers and said it wasn’t.’

‘Maybe it’s not the link then,’ said Harper. ‘Keep at it, Denise. We’ll find something.’

‘As far as I can see, Tom, it’s all a grand delusion. Section 88 are finished, but this guy is carrying on. He fed his delusion through this assumed identity of Josef Sturbe and I’m worried about what he’s going to try next.’

‘We all are,’ said Harper.

Dawn became morning. Harper watched from the surveillance truck as the rest of the crowds left, followed by the media trucks, all empty-handed.

The police operation started to pack up and leave. Harper was the last to go. He jumped off the Command Truck and waited as it pulled off. By 7.30 a.m., there was nobody left.

Harper stood with Eddie, cold, hungry and tired. They looked around the empty lot. Harper saw a single police truck standing in the street and moved towards it.

‘What is it?’ called Eddie.

‘Auxiliary Support Truck,’ said Harper.

Eddie shook his head. ‘Auxiliary Support. Amateurs, Harper. They went home and left their truck!’

Harper walked across. There was no one in the cab. ‘The keys are still in the ignition,’ he said.

‘Some part-time nobody went home,’ said Eddie. ‘He’s going to wake up soon and think he’s missing something.’

Harper walked around it. It was on his second round that he spotted the tube running up the side of the van. It was painted the same color as the vehicle. He raised his eyes. The tube went right up to the roof. Harper put his foot on the big back wheel and jumped up. His hands caught the top of the truck and he pulled himself up.

‘What you doing?’ said Eddie.

Harper didn’t reply. He peered over the top of the truck. The tube ran right across the roof, to where there were three air vents. Harper looked at each. Two had been sealed up. The third one had a small metal nozzle sticking up from it. The tube crossed the roof and joined to the nozzle.

Harper dropped down from the truck and crouched beside it. He pulled out his flashlight and shone it under the chassis. The tube ran right under the truck and connected to the exhaust pipe.

Harper stared, the horrific truth coming to him in a flash. He stood up. Eddie moved across. ‘What is it?’

Harper let the flashlight do the talking. He ran the beam over the tube, all the way from the exhaust pipe to the roof. Eddie watched the light. ‘Jesus Christ.’ He crossed himself.

‘Evil,’ said Harper. ‘This is what it looks like.’

Harper put on a glove. ‘Don’t touch a thing,’ he warned Eddie as he pulled the keys out of the ignition and walked round to the rear of the truck. ‘I don’t want to open it,’ he said.

‘You think you should?’

‘What if someone’s unconscious in there?’

Eddie nodded and Harper pushed the smaller key into the lock. It turned easily. He twisted the handle and pulled open the truck doors.

Both men reared back as the exhaust fumes that saturated the inside of the van billowed out into the glossy night lights still burning in Union Square Park.

The blackened insides of the truck were dark like a cave. Harper held his arm across his mouth and nestled his nose and lips into the curve of his elbow. He raised his flashlight and pointed the beam through the smoke. The widening beam of light caught bright pink flesh. Harper’s light moved laterally. One, two, three, four, five. Their faces and hands glowed garishly from the carbon-monoxide poisoning; the skin was pricked and dimpled. Their bodies clung to each other, the whole image like some terrible vision of hell.