The whistle made her ears throb with pain and threatened to deafen; the good news was that it would also deafen him, so he wouldn’t sense her approach.

Smells of moist concrete, mold, mud. The sensation took Sachs back to her childhood, father and daughter at the zoo in Manhattan, one of the houses, reptile. ‘Amie, see that? That’s the most dangerous thing here.’

She’d peered inside but couldn’t see anything other than plants and rocks covered with moss. ‘I don’t see anything, Daddy.’

‘It’s a leeren Käfig.’

‘Wow. What’s that?’ Snake, she’d wondered. Lizard? ‘Is it dangerous?’

‘Oh, the most dangerous thing in the zoo.’

‘What is it?’

‘It means “empty cage” in German.’

She’d laughed, tossing her tiny red ponytail as she’d looked up at him. But Herman Sachs, a seasoned NYPD patrol officer, wasn’t joking. ‘Remember, Amie. The most dangerous things are the ones you can’t see.’

And now too she saw nothing.

Where was he?

Keep going.

Ducking and, with as deep a breath as she could take yet not choke on the mist in the air, she stepped through the cloud.

And she saw him. Unsub 11-5.

‘Jesus, Rhyme,’ she whispered, stepping closer. ‘Jesus.’

Only after some moments of hearing nothing but the wail and hiss of the water did she remember that the mike and camera were off.

The experts from Fort Detrick had helicoptered into town in all of forty-five minutes.

When the poison in question is sufficient to kill a high percentage of the population of a major US city, the national security folks don’t fool around.

Once it was clear that the unsub was not going to be shooting anyone, Sachs was politely but emphatically ordered out of the tunnel while eight men and women in elaborate self-contained biohazard suits went to work. It was clear from the start that they knew what they were doing. Fort Detrick, in Frederick, Maryland, was home to the US Army’s Medical Research and Materiel Command and its Medical Research Institute of Infectious Diseases. In effect, if the prefix ‘bio’ and the words ‘warfare’ or ‘defense’ were linked in any project of any kind, Fort Detrick was involved.

Rhyme’s voice clattered through the radio. ‘What, Sachs? What’s going on?’ She was standing, freezing, on the slushy sidewalk near Third, where she’d parked her Torino.

She told him, ‘They’ve secured the botulinum. It was in three syringes in a thermos. They’ve got them in a negative pressure containment vehicle.’

‘They’re sure none got into the water?’

‘Absolutely positive.’

‘And the unsub?’

A pause. ‘Well, it’s bad.’

Rhyme’s plan to have the city announce falsely that the water supply was going to be shut down had had one unexpected consequence.