By Steven Eke
BBC Russia analyst
The announcement by the FSB, Russia's security service, that it has opened "an espionage investigation" on the basis of Andrei Lugovoi's recent accusations, is an unexpected twist in an already highly convoluted story.
Andrei Lugovoi has denied the charges against him
Mr Lugovoi, whose extradition from Russia on suspicion of murder has been formally requested by the UK, made a series of serious accusations relating to the British secret services.
He alleged that Alexander Litvinenko had contacts with British intelligence, and that he had tried to recruit him to gather "compromising material" on President Vladimir Putin.
In separate interviews to the Russian media, Mr Lugovoi has alleged that he was "blackmailed", and his business interests "threatened" by the British authorities. So far, he has presented no evidence for any of this.
Source of authority
The British secret services do not comment publicly in response to accusations of this type. However, the Foreign Office insisted the Litvinenko murder was a criminal matter, and not a question of intelligence.
There is a now dominant faction in the Russian administration whose method is to bolster its own grip on power and influence over the country by fomenting anti-western sentiment
Mr Lugovoi made the accusations at a time of considerable antagonism between Russia and the West. One of the reasons behind the tension is the level of espionage, which both sides allege has returned to levels not seen since the Cold War.
The influence of the FSB, and Russia's other security services, can not be overestimated. They have arguably become the most important source of authority in the country.
They have provided the majority of candidates for appointment to senior bureaucratic posts over recent years. And they have had a central role in determining both the conduct and outcome of many of the most controversial trials.
Last year, the Russian secret services were further strengthened by amendments to Russian law, which effectively allow the assassination of "enemies" of the Russian state abroad.
Officials insist the changes were aimed at boosting the fight against terrorists, including the killers of Russian diplomats in Iraq. Critics say they gave the security services carte blanche to target political opponents too.
There is a now dominant faction in the Russian administration whose method is to bolster its own grip on power and influence over the country by fomenting anti-western sentiment.
This helps explain why the Russian security services have become so visibly engaged in a case Britain insists is purely criminal.
The approach is to whip up "spy-mania", and accentuate the alleged harm caused by Russia's "enemies" abroad. The question of how such a campaign looks from afar is irrelevant; it is intended, above all, for domestic consumption.