The call from Radio Echo Moscow came on Saturday, November 11, 2006: "Can you confirm that Alexander Litvinenko has been poisoned?"

I was in Paris en route to London, and I ­didn't know anything about it so I went on the Internet to check. The initial source of the report was FSB's Web site, ChechenPress.info, which announced that on November 1, Sasha had been poisoned, allegedly by the true FSB.

I reached Sasha on his cell phone. He was in a small community hospital in North London, not far from his home. He sounded vigorous.

"I was throwing up for three days before they took me to the hospital. The doctors think I ate bad sushi, but it's not that, I know."

"What about the Italian guy?" I asked. According to Zakayev's Web site, Sasha became ill after eating sushi with Mario Scaramella, of whom I had never heard before.

"Well, we were in the sushi bar together, so he could have slipped something into my soup."

My initial reaction was that this was just too much. An Italian lacing his miso soup with poison? Surely it was just a case of bad sushi, I thought.

I called Sasha's wife Marina. She said doctors had found a bacterium in his system, which she "could not even begin to pronounce. They gave him some antibiotics."

"Okay, then. I will be in London tomorrow."

It sounded so innocuous. I did not see Sasha until Wednesday, November 15. He was still feeling lousy, and I began to be slightly worried: two weeks is just a bit long for food poisoning.

What I saw when I arrived at Barnet Hospital did not make me feel better. They kept Sasha in an infection-safe environment. I had to put on plastic gloves and an apron before entering the ward, and refrain from touching him, to protect him from accidentally catching a bug from outside.

"He is neutropenic," the doctor said, meaning that his white blood cell count was down. This happens when the bone marrow stops producing cells needed to fight off infection. No food poisoning would cause such a symptom.

"Why?" I asked.

"We ­don't know. Theoretically it may be a virus, something like AIDS, or an unknown reaction to the antibiotic he received initially, or a large dose of some chemotherapeutic drug, or heavy irradiation. But he was not near any radiation source and has not received chemotherapy. And he is HIV-negative. Frankly, we are at a loss."

"We suspect foul play," I said. "Have you notified the police?"

"At this point the cause can be benign or sinister. We ­can't contact them until we are sure. ­We're waiting for a toxicology report."

Sasha looked thin and gray. He had not eaten for two weeks, subsisting on IV transfusions. But he was moving around the room, and he was in a fighting mood.

"The way it started, I thought I'd die," he reported. "But I immediately drank a gallon of water and made myself throw up, to clean the stomach. These morons, they ­didn't listen to me. When I told them I was poisoned by the KGB, they wanted to call a psychiatrist. You have to get it into the British press."

"I already called a couple of journalists. But no one will touch it without police or hospital confirmation. When toxicology arrives, ­we'll know for sure what's wrong with you."

By now, thanks to Sasha and Boris Berezovsky, I was an expert in publicizing unbelievable explanations of incredible events, and this one was the most incredible yet. On the other hand, a very ill man was in front of my eyes, and there was no better theory than poison.

"Tell me about the Italian."

"The Italian has nothing to do with it. I named him on purpose, as a trick. The real man is Andrei Lugovoy, but please keep it secret. I am trying to lure him back to London."

True to himself, Sasha was playing out another gambit. He was sure that Lugovoy, Boris's former head of ORT security, had poisoned him. After his illness was reported in Russia, Lugovoy called him from Moscow to wish him a swift recovery.

"I told Lugovoy that I suspect the Italian, to make him feel it's safe to come again, to finish me off," he smiled wryly.

Just about a year earlier, at a grand party Boris threw on his sixtieth birthday in a rented castle outside of London, we had shared a table: Sasha, Marina, Andrei Lugovoy, and I. At the time he barely registered in my memory; he was a shadow from the Russian past, one of two hundred guests. But as Sasha told me at the hospital, that party was the beginning of a surprisingly intense interaction between them. Back in Moscow they had never been close.

After having served fourteen months in prison, Lugovoy went into business and became immensely successful, benefiting from the new Russian prosperity caused by skyrocketing oil prices. His core enterprise was his security agency, which provided bodyguards to hundreds of nouveau riche Muscovites. He bragged to Sasha about his multimillion-dollar investments in the food and services industries. He suggested that they work together; Sasha could be his man in London. Surely there must be British security companies interested in the Russian market.

Sasha produced impressive references from the security companies he had been working with. Over the year they met two or three times. No real business had come of it, but the prospects seemed great. His last meeting with Lugovoy was on November 1, in the Pine Bar at the Millennium Hotel on Piccadilly, two hours after he went out with Mario Scaramella. Lugovoy was with another Russian, Sasha said, whom Sasha had not met before. "He had the eyes of a killer," he said. He knew the type.

The next morning, I went to the hospital with Boris, who like myself had initially discounted Sasha's illness as a stomach bug. Sasha was visibly worse. His hair had started falling out; he pulled a pinchful to demonstrate. He was suffering tremendously from an apparent inflammation of his gastrointestinal tract, all the way from his mouth, which was so painful he could barely talk or swallow, to his bowels. It was as if his insides had been burned by an unknown irritant. The doctors had started him on painkillers. They still did not know the cause of it all.

I contacted Prof. John Henry, the renowned toxicologist at St. Mary's Hospital, who had gained considerable fame, in the Russian universe at least, in 2004 when he diagnosed the poisoning of Viktor Yuschenko simply by looking at his face on TV. It was the substance called dioxin, he said, and indeed, some time later, lab analyses confirmed it.

I described the symptoms to Professor Henry over the telephone.

"Hair loss is a hallmark of thallium," he said. "But bone marrow malfunction sounds strange. Does he have muscle weakness?"

Thallium, a heavy metal, had been banned in the United Kingdom but was readily available as a rat poison in grocery stores throughout the Middle East. It acts by slowly destroying the outer shield of nerve cells. Survivors may have long-term neurological problems. A nurse in Qatar made headlines in the 1970s when, after reading Agatha Christie's novel The Pale Horse, she recognized a case of thallium poisoning that had baffled doctors. Thallium poisoning was the basis of a conspiracy theory swirling around the death of Yasir Arafat. Some say the CIA planned to embarrass Fidel Castro by sneaking thallium into his shoes, hoping it would cause his beard, eyebrows, and pubic hair to fall out.

On the strength of these stories and Professor Henry's guess, I finally persuaded a reporter, David Leppard from the Sunday Times, to see Sasha at the hospital. He realized, of course, that without objective confirmation of poison there was no story, but he came just in case the toxicology report proved foul play; then, by press time, he would have an exclusive for the Sunday paper. He interviewed Sasha in Barnet Hospital on Thursday evening.

Alexander Litvinenko had been poisoned—but not by thallium. In tomorrow's excerpt, the trail of Polonium-210 leads to Russia.