"No," he said emphatically. He sat down on his bed. "I would never do that. You've got to believe me."

"You know that's why you're here," I said. "You know that your father believes you're guilty."

"I'm here because of my father," Billy said, flipping his hair out of his eyes again, "but he doesn't believe I hurt Brooke."

"Then why would he send you?" I asked.

"Why? Probably because he's the one who did it. And he isn't about to take the blame. He never does."

"Why would your father kill your sister?" I asked.

"He never wanted one baby girl, let alone twins," Billy said. "He wanted my mother to have an abortion."

"How do you know that?"

"He was always screaming at her to get one." Billy squinted at me, as if remembering. "I'd wake up in the middle of the night and hear him yelling, 'Get rid of them! Stop thinking about yourself all the time! Get rid of them or get out!' Mom would cry and curse him and say she'd do it. I think she even went to an appointment once at one of those family planning places. But she never went through with it."

I remembered Laura Mossberg's comment that Billy's extraordinary intelligence might just make him a more cunning liar. "Why tell me all this?" I said. "Why didn't you tell Dr. Mossberg or the police?"

"You're the only one who asked," he said. He swallowed hard. "I got to take a chance, sometime, on somebody." Our eyes locked. "Also, I figure I got just about nothing left to lose."

I found Laura Mossberg back at her office. She invited me in. I took the seat near her desk.

"Well," she asked. "Any breakthroughs?"

"Only one, if you want to call it a breakthrough," I said. "He insists he didn't hurt his sister."

"He answered you directly about that?"

"Yes," I said. "He did."

"Did he point a finger at someone else?"

I couldn't be sure whether what I told Mossberg would stay with her or be passed on to Darwin Bishop. Billy was a minor, after all. His medical records were officially the property of his parents. And Darwin Bishop probably had an even more immediate pipeline to the goings-on at Payne Whitney, through his friend, the medical center's CEO. "No," I said. "He didn't have any theory about Brooke's murder."

She nodded. "So, let me ask you the same question you put to me: Do you believe him? Could someone else be responsible?" She toyed with the pearls hanging around her neck.

"I have no reason to believe him right now," I said. "His psychological profile, his prior history of violence, his lying on the standardized tests you administered here-all of it puts whatever he says in grave doubt. I suppose the shocking thing would be if he admitted the crime."

"Agreed," she said. "But you seem troubled. What's on your mind?"

I knew I was sitting with someone trained to listen to the music between spoken words. "I feel for him," I said, hoping that would be a sufficient explanation. "Like his father told me, Billy isn't evil, he's ill."

"And on that score, could you be helpful to him in court? Does he meet the criteria for an insanity plea?" Mossberg asked.

"He certainly has a history of terrible trauma," I said, "going all the way back to his childhood in Russia, witnessing the murder of his parents. A case could be made that he lost the emotional ties that bind the rest of us. Without empathy, without conscience, he might not have any brake on his primal feelings-including being pathologically jealous of new children in the family. He may have lashed out as a reflex, rather than a premeditated act. To put it in" legal terms, he may 'lack the substantial capacity to conform his behavior to the requirements of the law.' "

"That rings true," she said. "His psychological testing would support that."

I looked into Blue Dog's golden eyes. "Just out of curiosity," I asked, "did Billy have a physical examination when he was admitted?"

"He refused," Mossberg said. "We didn't see a reason to press him on it. He's been quite healthy-from a medical standpoint-according to his father." She paused. "Is there anything in particular you're concerned about?"

"Billy has quite a few welts on his back," I said, looking at Mossberg. "Some are scarred over. Others are fresh."

She nodded. "That would be consistent with what Mr. Bishop told me," she said. "Apparently, Billy has the habit of whipping himself with a belt-along with his cutting, biting, and hair-pulling. I've understood all of that as an outgrowth of his self-hatred. He makes attempts to channel his violence inward, but it inevitably spills over, and he strikes out at others."

"Mr. Bishop hadn't told me about the belt," I said. "Just the other behaviors."

Mossberg shrugged. "Maybe it slipped his mind. He may not have thought it was as important to let you know, given that you wouldn't normally be doing a physical examination."

"That's probably right," I said. It was equally possible that it had "slipped" Darwin Bishop's mind because he didn't think I would find out about it.

"How did he come to show you his back to begin with?" Mossberg asked.

"Very much by accident," I fibbed. "He took off his shirt to intimidate me. He's a strong kid and he looks it. For a minute there I thought he might attack me."

"I'll keep that at the front of my mind," she said. "I bruise easily." She winked. "Is there any other way I can be helpful to you?"

"Will you be assembling Billy's other medical records?" I asked. "I understand he's been treated by other psychiatrists."

"We've sent out the relevant requests," she said. "I'll be sure to call you with anything we get our hands on."

"That could be a big help," I said.

I grabbed a quick lunch at a greasy spoon and hailed a taxi. I was anxious to get my hands on information about Darwin Bishop's 1981 conviction for assault. I'd had luck getting case records before at the Office of Court Administration, way downtown on Beaver Street, just below Wall Street, a couple blocks from Battery Park.

"Let's take Second Avenue, headed downtown," I told the cab driver. I opened the window a few inches to let out the odor of stale smoke that was making me hold my breath.

"Why Second?" he said, without turning around. "The FDR. Faster." He had a European accent I couldn't quite place. Maybe Russian.

I glanced at his photo ID, mounted to the dash, next to a white plastic Jesus. His name was Alex Puzick. He looked about sixty years old. His eyes were weary. His face was half-shaven. He wore a white shirt that had yellowed at the collar and along the shoulder creases. "I want to make a quick stop at the River House," I said. "It won't take me more than a minute."

He answered by throwing the car into drive and barreling across 67th Street, then down Second Avenue.

As I half-watched the endless parade of copy shops, boutiques, groceries, and electronics stores, my mind kept wandering to Tess Bishop, Brooke's surviving twin. Because I wasn't more than fifty-fifty on Billy's guilt. And that left even odds that a killer was still loose on the Bishop estate.

I wondered if I could move the Department of Social Services office on Nantucket to take custody of the child until the murder investigation was further along. But the likelihood of DSS intervening, given the District Attorney's exclusive focus on Billy, was slim.

The key might be a direct appeal to Julia Bishop to place her daughter in a safer environment. I knew that wouldn't be without risk; if she shared my suspicions with her husband, he would almost certainly shut the door completely on me-and North Anderson.

I was still weighing the idea of talking openly with Julia when the cab driver glanced over his shoulder. "Live here?"

"No," I said. "I live outside Boston."

"What brings you?"