Commander Penn handed me a high-powered flashlight as we were escorted down by four officers who were husky in Kevlar and coats and reflective vests. A train blew by in a stream of liquid steel, and we inched our way along a catwalk that led us into dark catacombs littered with crack vials, needles, garbage and filth. Lights licked over hobo camps set up on pallets and ledges within inches of rails, and the air was fetid with the stench of human waste.

Beneath the streets of Manhattan were forty-eight acres of tunnels where in the late eighties as many as five thousand homeless people had lived. Now the numbers were substantially smaller, but their presence was still found in filthy blankets piled with shoes, clothes and odds and ends.

Grimy stuffed animals and fuzzy fake insects had been hung like fetishes from walls. The squatters, many of whom the Homeless Unit knew by name, had vanished like shadows from their subterranean world, except for Freddie, who was roused from a drugged sleep. He sat up beneath an army blanket, looking about, dazed.

‘Hey, Freddie, get up.’ A flashlight shone on his face.

He raised a bandaged hand to his eyes, squinting as small suns probed the darkness of his tunnel.

‘Come on, get up. What’d you do to your hand?’

‘Frostbite,’ he mumbled, staggering to his feet.

‘You got to take care of yourself. You know you can’t stay here. We got to walk you out. You want to go to a shelter?’

‘No, man.’

‘Freddie,’ the officer went on in a loud voice, ‘you know what’s happened down here? You heard about Officer Davila?’

‘I dunno nothing,’ Freddie swayed and caught himself, squinting in the lights.

‘I know you know Davila. You call him Jimbo.’

‘Yeah, Jimbo. He’s all right.’

‘No, I’m afraid he’s not all right, Freddie. He got shot down here tonight. Someone shot Jimbo and he’s dead.’

Freddie’s yellow eyes got wide. ‘Oh no, man.’ He cast about as if the killer might be looking on - as if someone might want to blame him for this.

‘Freddie, you seen anybody down here tonight you didn’t know? You seen anybody down here who might have done something like that?’

‘No, I ain’t seen nothing.’ Freddie almost lost his balance and steadied himself against a concrete support. ‘Not nobody or nothing, I swear.’

Another train burst out of the darkness and blew past on southbound tracks. Freddie was led away and we moved on, sidestepping rails and rodents moiling beneath trash. Thank God I had worn boots. We walked for at least ten minutes more, my face perspiring beneath my mask as I got increasingly disoriented. I could not tell if round bright lights far down the tracks were police flashlights or oncoming trains.

‘Okay, we’ve got to step over the third rail,’ Commander Penn said, and she had stayed close to me.