"But it's possible with enough expertise," Alex insisted. "Did you come up with any other options?"

"Chemicals," said Chris, making steadily for Jewish Hill. "As I suspected, the toxins known to cause cancer are some of the most persistent on the planet. You put one nanogram of dioxin into somebody, it'll be there on the day they die. Detailed toxicology studies on autopsy would turn up things like that very quickly. As for volatile compounds like benzene, which you mentioned the second time we met, you'd have the same problem you have with radiation. Using enough to reliably kill people would almost certainly cause acute illness. So basically, as a class, chemicals are a less reliable oncogenic murder weapon than radiation, but more likely to get you caught. I suppose-"

"I'm sorry. Oncogenic?"

"Cancer-causing," Chris clarified.

"Sorry. Go on."

"I suppose someone could come up with an untraceable oncogenic poison-the CIA or the army, I mean-but in that case you'd have almost no practical hope of discovering it."

Alex looked thoughtful. "But it's something to consider. I haven't been profiling intel or military officers as suspects, but maybe that's a realistic option."

"Not around here. Fort Detrick, Maryland, is where they keep the germs and toxins. You really need to talk to an expert, Alex. And I don't mean a garden-variety hematologist. You need somebody from NIH or Sloan-Kettering or Dana-Farber." Chris stopped and watched a half dozen butterflies flitting around a bush bursting with purple flowers. One had marks on its wings that looked almost psychedelic, rounded spheres of electric blue. "M. D. Anderson is probably the closest place."

"That's Houston?"

"Right. Seven hours by car."

Alex held out her hand, and one butterfly danced around her extended finger. "And what do I ask these experts? What would you ask them?"

Chris started walking again. "If we dispense with radiation and chemicals in our little hypothetical, that leaves only one possibility I know of. And it's a biggie."

"What is it?"

"Oncogenic viruses."

She turned toward him. "A professor I spoke to last week mentioned viruses, but a lot of what he said was over my head."

"Do you know anything about retroviruses?"

"Only that AIDS is caused by one."

"Reverse transcriptase?"

Alex looked embarrassed.

"Okay. Some viruses in the herpes family are known to cause cancer. And there's at least one retrovirus that's known to be oncogenic. If there's one, there are probably more. There are theoretical models about this stuff, but it's not my area. I was thinking of calling my old hematology professor from medical school. Peter Connolly. He's up at Sloan-Kettering now. He's done groundbreaking work on gene therapy, which actually uses viruses to carry magic bullets to tumor sites. It's one of the newest forms of cancer therapy."

"From Jackson, Mississippi, to New York?"

Chris laughed again. "It happens. Didn't you know that the first heart transplant in the world was done in Jackson?"

"I thought that was in Houston, too."

"The Jackson transplant was done on a monkey. But the technology was the same. The difficulty was the same. Kind of like the first space shot. Michael DeBakey and Alan Shepard-monkeys helped blaze the trail for both of them."

They had reached Jewish Hill at last, but as they started toward its forward precipice, and the immense vista it offered, Chris glanced at his watch.

"Alex, I hate to say this, but I've got to run. Ben's at a birthday party, and with Thora gone, I've got to pick him up."

She smiled. "It's okay. We can jog back to the car."

They started trotting downhill, but Alex clearly didn't intend to squander her remaining time with him. "I've wondered about someone simply injecting tumor cells from a sick person into a healthy one. I saw that done with mice on the Discovery Channel."

A little knowledge was a dangerous thing, Chris reflected. "They can do that because the mice used for cancer research are either nude mice, which means they have no immune systems, or because they're genetic copies of each other. Clones, basically. That's like injecting cells from a tumor in my body into my identical twin. Sure, those cells would grow, or they'd have a chance to, anyway. But if I injected cells from my tumor into you, your immune system would quickly wipe them out. Very violently, too, on the cellular level."

"Are you positive? Even with really aggressive tumors?"

"I'm pretty certain. Even with what we call undifferentiated tumors, those cancer cells began as part of a specific person, from their unique DNA. Any other person's immune system is going to recognize that foreign tissue as an alien invader."

"What if you somehow beat down your victim's immune system beforehand?"

"You mean like with cyclosporine? Anti-organ-rejection drugs?"

"Or corticosteroids," Alex suggested.

She had been doing her homework. "If you compromised someone's immune system sufficiently to accept cancer cells from another person, they'd be vulnerable to all sorts of opportunistic infections. They'd be noticeably sick. Very ill. Do the medical records of your victims show strange illnesses before their cancer diagnosis?"

"I only have access to the records of two victims. But, no, those records don't show anything like that."

The Corolla was forty yards away. Chris cut across the grass, picking his way between the tombstones. "If you had the records of every victim, you might be able to learn a lot. You could really move this thing forward."

Alex stopped beside a black granite stone and looked at him with complete candor. "I feel so inadequate in this investigation. I mean, my genetics stops at the high school level. Mendel and his peas. But you speak the language, you know the experts we need to talk to-"

"Alex-"

"If I can get hold of the other records, will you consider helping me analyze them?"

"Alex, listen to me."

"Please, Chris. Do you really think you're going to be able to avoid thinking about all this?"

He grabbed her hands and squeezed hard. "Listen to me!"

She nodded almost violently, as though aware she had crossed some line.

"I'm not sure what to do yet," he said. "Everything that's happened is swirling around inside me, and I'm trying to come to terms with it. I'm working on it, okay? In my own way. I am going to call my friend at Sloan-Kettering tomorrow."

Alex closed her eyes and exhaled with relief. "Thank you."

"But right now I need to pick up Ben, and I don't want to be late."

"Let me give you a ride to your truck. Where is it?"

He dropped her hands. "At home."

"Home! It'll take you an hour just to get there."

"A half hour."

"You have to let me take you."

He walked the rest of the way to the car, and she followed. "I need to be alone, Alex. I've had all I can take for now." He unlocked his bike and took it off her rack. "I'm going to call in a prescription for you, to help you sleep."

"Those things don't work for me. Not even Ambien."

"I'm going to give you Ativan. It can't not work, unless you're already addicted to it. If you don't sleep, you'll still relax. Is Walgreens pharmacy okay? That's near your motel."

"Sure."

He climbed onto the Trek and held out his hand. When Alex took it between both of hers, he felt her hands shivering.

"Promise me you'll be careful," she implored. "Stay away from the traffic."

"I'll be fine. I do this all the time. Now, let me go. We'll talk later."

"Tonight?"

"Maybe. By tomorrow for sure."

"Promise?"

"Jesus."

Alex bit her bottom lip and looked at the ground. When she raised her head, the sclera of her eyes were shot with blood. "I'm out on the edge here, Chris. You are, too. Only you don't know it."