AT home in the suburbs of Boston, Yuri Felshtinsky reflects that every Russian emigre must shape the nature of exile for himself. What is important is where the soul resides. The other day that sentiment compelled him to travel to London, against FBI advice about his safety. But the reason for the risk was simple, he says: to honour the memory and courage of his friend, the murdered Alexander Litvinenko.

Felshtinsky, a historian specialising in the Russian secret services, is the co-author of Blowing Up Russia, the book both men struggled to publish for six years. In that time they learned from insiders that they, like the investigative journalist Anna Politkovskaya, were assassination targets. Everything about Politkovskaya, a trenchant critic of the Kremlin's war in Chechnya, was a reproach to the authorities who sought to muzzle her. Last October she was killed by gunfire outside her Moscow apartment. A month later, Litvinenko's horrific death in London would turn his and Felshtinsky's book into hot property. It has just been published in Britain, and by the end of the year its attack on Litvinenko's former spymasters will be available in 20 languages.

Just this week, another Russian, journalist Ivan Safronov, a military affairs writer for the daily Kommersant, died after falling out of a stairwell window in what some media said could have been murder. He was said to be planning to publish evidence of Russian attempts to sell weapons to Iran and Syria via Belarus.

"Before Alexander was killed people were not ready to accept that the story of contemporary Russia is so dark," says Felshtinsky. "Those who knew of the accusations would say: "If you are correct that the old-style terror has returned, how are you still alive?" Now we have the terrible proof that Alexander was right."

Our conversation took place between Felshtinsky's visit to his London publishers and a rendezvous with Scotland Yard to elaborate on his friendship with Litvinenko. He also met that pivotal figure in the London/Russian community, Boris Berezovsky, the exiled billionaire and former member of Boris Yeltsin's inner circle, who is daggers drawn with the ex-KGB boss he helped to power, President Vladimir Putin.

The oligarch, who amassed his fortune after the fall of the Soviet Union, is wanted by the Kremlin on alleged fraud charges, but Russia's demands for Berezovsky's extradition have been refused twice by Britain. In London he was Litvinenko's patron after the former secret service officer fled Moscow, having warned Berezovsky that he had been instructed to kill him. But before his escape, Litvinenko went public with that claim. In 2001 he was granted British asylum with his wife and son.

When Litvinenko was dying, Berezovsky had already accused the Kremlin of involvement, a claim Russia initially dismissed as nonsense. But Felshtinksy also has no doubt where the guilt lies, and insists that the parallel murder investigation now being conducted in Russia is simply window dressing for the west. "They are not seriously committed to solving the crime. If they were, they should start with arresting the two major suspects, Andrei Lugovoi and Dmitri Kovtun."

These are the Russian businessmen with whom Litvinenko had tea at the Millennium Hotel in London on November 1, the day he was poisoned with polonium-210. Traces of the then unidentified material were found at the hotel, and at other locations where Lugovoi and Kovtun also visited. But any extradition request from Britain for the two is likely to be refused on grounds that it contravenes the Russian constitution.

"These two men, who could help provide the complete picture about the murder, are walking around Moscow with no problems. Both of them seem quite happy over there."

Litvinenko had been an officer in a secret security unit dedicated to rooting out corruption, but among his claims was the charge that the reconstituted KGB - known now as the FSB - fabricated terrorist attacks to launch the second Chechen war, and thus ensure a landslide presidential victory for its old boss, Vladimir Putin.

It was in 1978 that the young Felshtinsky, equipped with a history doctorate, emigrated to the US. Twenty years later he returned to acquaint himself with the New Russia, and it was then that he met Litvinenko. "Alexander was preparing for that momentous press conference where he declared that top FSB generals were giving illegal orders to junior officers. He knew that he would be fired for such "betrayal". He knew, too, that he would be punished and that not even Berezovsky would be able to save him."

By 2000, Felshtinsky had started to research the explosions which, in 1999, had ripped open apartment buildings in several Russian cities, claiming more than 300 lives. "I came to the conclusion that these terrorist' acts were, in fact, conducted by the Russian secret service to discredit the Chechens. But there were still many things that I could not understand and I needed Alexander's help."

From that moment the two men's collaboration began, resulting in the Blowing Up Russia manuscript. Berezovsky was instrumental in providing the money for the book and its accompanying documentary, Assassination of Russia. In December 2003, Litvinenko and Felshtinsky arranged for 5000 copies of the book to be printed abroad, a consignment which was intended for a supportive bookseller in Moscow. But the shipment was confiscated by the FSB and the work was officially banned.

Several chapters had already appeared in a special edition of Novaya Gazeta, the newspaper on which Politkovskaya worked. But by the end of that year three of the people involved in the exercise had been killed. Vladimir Golovlyev and Sergei Yushenkov, both members of the Duma (state parliament), were gunned down in 2002 and 2003. In 2003 Yuri Schekochikhin, also a member of the Duma and deputy editor-in-chief of Novaya Gazeta, was poisoned.

Our conversation turns to the question of Felshtinsky's own safety. "The first signal of danger came in November, 2002. This was an e-mail sent by a former FSB officer, now in prison in Moscow because he went against the system. He sent an e-mail to Alexander telling him the decision was made to kill him, and also that someone had been sent to Boston to check out the best way to assassinate me."

Felshtinsky had previously been in London last October to see Litvinenko and Berezovsky. By chance, on October 12, he bumped into Lugovoi and Kovtun in the street at Piccadilly, which, he says, is of interest to Scotland Yard because the authorities here have no record of the men being in Britain then.

But what of the moral consideration that, in pursuing his work on Litvinenko's behalf, Felshtinsky may also be endangering his family? "Security is more of an issue for my wife and children than me. They are frightened. Before I came to London they asked me to promise not to drink tea or take meals with strangers, and the FBI advised me not to travel. But what can you do to be safe against improbable crimes? I always thought that a US passport was my security. Now I cannot be sure.

"Even if Putin is not guilty of giving the direct order, he is responsible for creating a system which by law allows the FSB to kill people whom it considers to be an enemy of the Russian state."

Felshtinsky last spoke to Litvinenko on his mobile on November 8. He was calling from America and by then his friend was in hospital. "He was convinced that the FSB were involved but he believed the worst was behind him."

Litvinenko died on November 23, only hours after the lethal polonium-210 had finally been detected in his urine.

But why would his attackers plan such a grotesquely theatrical form of punishment?

"It's the old KGB mindset. Killing traitors must be carried out in a way that spreads maximum fear."

A citizen of the US for almost 30 years and the author of many Russian studies, Felshtinsky still feels Russian by culture.

Yet when he left the Soviet Union, he knew he'd lost the Motherland. Then, in 1991, when Communism collapsed, Russia refound herself, and the world rejoiced in a victory without bloodshed.

But today the deaths of Anna Politkovskaya, Litvinenko and now the death of Ivan Safronov signify a wider fear for Yuri Felshtinsky than one of personal safety. It's as if he and all of us, he says, are losing Russia to darkness once again.

  • Blowing Up Russia by Alexander Litvinenko and Yuri Felshtinsky is published by Gibson Square, £14.99.