Rhyme couldn’t argue with that attitude. What could be more frightening than to think you were alone and safe in the lower levels of your home or office or a public building and learn that you weren’t alone at all; you had lethal company. Like a moist, venomous centipede uncurling under the blankets of your bed as you slept.

Sachs had brought Braden Alexander’s clothing too. Cooper went through each item carefully but the water had eradicated all trace – if there’d been any in the first place, which was unlikely, Sachs said, because the contact between the two men had been minimal. The handcuffs revealed no trace and, like the others, were generic.

Cooper ran other samples of swabs from the implant bag. Most were negative. But finally he had a hit. Reading from the computer screen, he said, ‘Hypochlorous acid.’

Rhyme looked over the mass spectrum. ‘Curious. It’s pure. Not diluted.’

‘Right.’ Cooper reached under the face shield and shoved his glasses higher on his nose. Rhyme wondered, as he often did, why he didn’t get frames that fit.

Hypochlorous acid – a form of chlorine – was added to New York City drinking water, as in most cities, for purification. But because this sample was undiluted, it had not come from the flood that had destroyed the Belvedere parking garage crime scene. This was the form of the chemical in its pure state, before it was added to the water system.

Rhyme said, musing, ‘It’s a weak acid. At higher levels, I suppose, it could be deadly, though. Or maybe he just picked it up because he was near one of the boxes that dispense it into the water supply. Sachs, at the first or second scenes, in the tunnels? There were water pipes, right?’

‘Water and, in one, sewage.’

‘Incoming and outgoing,’ joked Pulaski. Drawing laughs. From everyone except Rhyme.

‘Any other pipes – maybe some feeding chlorine into the mains?’

‘I don’t remember.’

‘I want to find out. If this chlorine is from tap-water purification it’s not helpful. If it’s from a poison he’s planning to use, then we can start checking sources.’ Rhyme called up the pictures from the first two crime scenes. ‘Let’s get somebody back to the scenes and find out if there’s a feeder line for the chemical.’

Sachs asked, ‘Do you want Crime Scene to search?’

‘No, just a uniform’ll be fine,’ Rhyme said. ‘Anybody. But soon. Now.’

Sachs called Dispatch and had patrol cars sent to each of the two previous crime scenes, with instructions on what to look for.

Twenty minutes later Sachs’s phone rang. She answered, then hit speaker.

‘Okay, Officer, you’re on with me and Lincoln Rhyme.’

‘I’m at the Elizabeth Street scene, Detective. The Chloe Moore homicide.’

‘Where are you exactly?’ Rhyme asked.

‘In the tunnel, next to the crime scene lamps and battery packs.’