Mrs. Delaney’s hands were shaking as she knelt to gather the towels. “It’s nothing,” she said too quickly. “The vents, they get dusty, that’s all.” But I had spent eight years learning to read the faces of people hiding things, and hers was a confession. I crouched by the decorative grate and looked closer. The sweet, artificial smell was not dust. It was chemical, and it was strongest exactly where the boys slept, funneled directly into their two adjoining rooms through a vent that served no other part of the house. Lorenzo watched me, and I could see the terror dawning as he began to understand what I was suggesting.

That night, after the boys were asleep, I did what no specialist had done. I stopped looking at Matteo and Marco and started looking at their environment. I traced the ventilation duct from the grate back toward its source, and I found something that did not belong: a small device, tucked inside the ductwork, slowly releasing vapor on a timer. It was not a household malfunction. Someone had installed it. Someone had been poisoning two little boys, a slow, careful dose released only into their bedrooms, only while they slept, for almost twenty months. And Mrs. Delaney’s face by that vent told me she knew who.