Dmitri V. Kovtun arrived in London for the first time in his life last Oct. 16. He dropped his bags off at a hotel near Piccadilly Circus and immediately went to meet, also for the first time, Alexander V. Litvinenko, the former K.G.B. officer whose killing by radioactive poisoning five weeks later became one of the most celebrated crimes of the post-cold-war era.

British investigators quickly zeroed in on Mr. Kovtun and an associate of his, Andrei K. Lugovoi, who both met with Mr. Litvinenko on Nov. 1, the day he fell ill. But Mr. Kovtun says they have it backward, maintaining that Oct. 16 was the day that Mr. Litvinenko exposed him to the poison, polonium 210. “I am far from thinking that something was premeditated,” Mr. Kovtun said. “I think things that were not premeditated were happening.”

Much uncertainty still shrouds Mr. Litvinenko’s death on Nov. 23, at 43, but Mr. Kovtun’s version — outlined in his most extensive and detailed interview, and impossible to verify independently — illustrates the starkly divergent view of the Litvinenko affair as seen from Moscow. It also suggests that sorting out the truth may ultimately be impossible, given the complex, secretive web of associations that bind Russia to its willing and unwilling exiles in London.

In British news media accounts not disputed by Scotland Yard, investigators have focused on a meeting that Mr. Kovtun and Mr. Lugovoi had with Mr. Litvinenko on Nov. 1. Together, they have been portrayed as secret agents sent to avenge Mr. Litvinenko’s betrayal of the K.G.B.’s domestic successor, the Federal Security Service.

Here in Russia, by contrast, prosecutors are investigating what they called an attempted murder of Mr. Kovtun from polonium exposure. (The extent of Mr. Lugovoi’s exposure is unclear.) In their few statements, prosecutors have suggested the possibility that Russian tycoons living in exile, including those who once ran Yukos Oil, ordered Mr. Litvinenko’s killing and, evidently, tainted Mr. Kovtun in the process.

Mr. Litvinenko’s relatives and associates abroad, in turn, say the Kremlin or security services ordered Mr. Litvinenko’s killing and are now trying to muddy public perceptions and hamper justice.

Whatever the truth of the case, Mr. Kovtun and Mr. Lugovoi, old schoolmates, friends and business associates, are at the center of what happened in London beginning the day that Mr. Kovtun arrived, traveling with Mr. Lugovoi and fulfilling a dream from his days of childhood English lessons “to see Westminster Abbey and other things” in London.

Everywhere they went on Oct. 16 — Erinys, an international security company on Grosvenor Street; Itsu, a sushi bar on Piccadilly, and the Best Western Premier Shaftesbury Hotel near Piccadilly Circus — later showed traces of polonium 210, according to British health officials. So did the Parkes Hotel, where they checked in the next day, unhappy with their first choice of accommodations.

Mr. Kovtun, who is 41, spoke in Mr. Lugovoi’s office on the second floor of the Radisson SAS Slavyanskaya, one of Moscow’s fanciest hotels. Mr. Lugovoi, 40, spoke in a separate interview there, and he also went to lengths to challenge the perception that has taken root in the West.

Mr. Kovtun is the only person ever officially identified — by a prosecutor in Germany — as a possible suspect, specifically a suspect in the unlawful handling of polonium while he visited his former wife in Hamburg from Oct. 28 to Oct. 31, before returning to London. He denied that.

Whatever the source, however, the traces of polonium followed Mr. Kovtun back to Moscow aboard a British Airways flight and to Germany, where he had served as a captain in the Soviet Red Army’s Main Intelligence Administration. He said he never served in the K.G.B. or its domestic successor.

After the Soviet Union collapsed, he remained in Germany and married a German. Although now divorced, he said, he and his former wife remain close, and she has agreed to meet with Russian investigators.

“When they began to say the address where traces of polonium were found, we realized that we visited those addresses only on the 16th and the 17th,” Mr. Kovtun said, referring to his first visit to London in October. “I thought my contact was there, since they found it in Germany. It could have only been brought from there. And when I realized those traces stayed for a long time, I thought they could have only come from there. I had never had any contact with polonium or with any radioactive substance.”

Mr. Kovtun said he could not explain how he was exposed. Nor would he speculate as to whether he believed Mr. Litvinenko had already been exposed somehow or whether he was carrying the material.

Nuclear experts said that if Mr. Litvinenko had absorbed a lethal dose on Oct. 16, the symptoms would have appeared almost immediately. That did not happen until the night of Nov. 1, after his meeting with Mr. Kovtun and Mr. Lugovoi.

Mr. Kovtun runs Global Project, a business consulting company he founded after returning to Moscow from Germany in 2003. It specializes in helping foreign companies — including some in Britain — to invest in Russia. Mr. Lugovoi, like Mr. Litvinenko, is a veteran of the K.G.B. department that guarded Soviet and later Russian senior leaders. Both went on to work closely with Boris A. Berezovsky, a billionaire tycoon. Mr. Lugovoi now owns Ninth Wave, a security company. On their first visit in October, Mr. Kovtun and Mr. Lugovoi said, Mr. Litvinenko seemed eager to introduce the Russians to his business contacts in London, including those at Erinys.

Mr. Kovtun said he did not have a favorable first impression of Mr. Litvinenko. “He was very politicized,” he said. “If he had a chance to talk about politics, he would do it willingly. And he spoke of absurd things.”

He did not elaborate on the subjects of Mr. Litvinenko’s talks, but he suggested that they included current affairs in Russia. Nevertheless, the three men met again on Oct. 17, having lunch at a Chinese restaurant.

After his visit to Germany, Mr. Kovtun returned to London on the morning of Nov. 1 aboard a plane belonging to an airline, Germanwings, that did not test for contamination, German officials have said.

He and Mr. Lugovoi did not plan to meet Mr. Litvinenko on that day, but they said that Mr. Litvinenko called them insistently on Nov. 1 to arrange a meeting. Mr. Kovtun said he had been having meetings at an investment company called Eco3 Capital, whose address is listed as 58 Grosvenor Street, a short walk from the Mayfair Millennium Hotel, where the Russians were staying.

British health officials said last November that polonium traces were also found at 58 Grosvenor Street. A representative of Eco3 Capital declined to discuss Mr. Kovtun’s visit, except to say that his business had been with a client, not directly with the investment company.

The three men agreed, at last, to meet later that afternoon at the Pine Bar in the Mayfair Millennium, where traces of polonium were found and seven members of the bar staff were exposed to small, nonfatal doses. Traces were also found in rooms, apparently those occupied by the Russians.

Mr. Lugovoi and Mr. Kovtun both noted that British and Russian investigators had cautioned them not to discuss the details of the investigation itself. In particular, both were wary of discussing what happened in the bar — including whether Mr. Litvinenko drank anything — but they described the meeting as short and largely unnecessary, given that they were planning to meet the next morning.

Mr. Kovtun described him as agitated. He said he looked unwell. “We did not speak with Litvinenko a long time, but he looked strange, and he was sitting next to me,” he said. “He kept talking. He didn’t close his mouth.”

When their names first surfaced, both men came forward and volunteered to meet with British investigators. They met with officials at the British Embassy in Moscow on Nov. 23, a few hours before Mr. Litvinenko died and before specialists determined that he had been exposed to polonium. Traces of polonium were later discovered at the embassy.

Mr. Kovtun went to the hospital for a test that showed he was “seriously polluted” with polonium, though he would not say exactly how much, citing his agreement with British and Russian investigators. He was treated and feels fine now, he said, dismissing as a lie a report in December by the Interfax news agency that he had slipped into a coma.

He did shave his head, prompting a flurry of speculation about his health when he appeared in a television interview, but he said he did it as a precaution since radioactive material could linger in his hair. His hair is growing back.

Both men said they had cooperated extensively with British and Russian investigators, one of whom, Mr. Lugovoi said, assured him that he was not a suspect. “Andrei, you are only a witness,” he said he recalled the investigator saying. Other witnesses have included Mr. Lugovoi’s family members and his personal secretary in Moscow, who never went to London.

Investigators in London and Moscow declined to discuss the case, but Scotland Yard did announce that it had presented the case to the Crown Prosecution Service, where officials will decide whether to bring charges. Those could come at any time, or never.

The Russians, meantime, said they were victims of circumstance, of bias and preconceptions and of the poison that killed Mr. Litvinenko.

“I want you to understand one thing,” Mr. Lugovoi said. “Myself and Dmitri Kovtun, we consider ourselves an injured party.”

Steven Lee Myers reported from Moscow, and Alan Cowell from Moscow and London.