There is one consolation for the team of Scotland Yard detectives trawling Moscow for evidence in the murder of Alexander Litvinenko — it is unseasonably warm. Otherwise, their work has been made so difficult, so quickly, that pessimists would say they had flown into a trap where they can do little but watch the trail go cold.

Having promised last week to co-operate fully with the British investigation, the Russian Prosecutor-General has thrown four separate obstacles in its way. He has told the visiting detectives that they may request interviews but only observe them, and then only if the interviews are granted. He has ruled out extraditing any Russian citizen for trial in Britain. He has announced his own investigation into the alleged attempted murder of two of Mr Litvinenko’s associates — who, as Russian citizens, provide a pretext for giving the Russian inquiry priority over the British one. And he has twice postponed interviews with the man Scotland Yard most wants to question.

That man is Andrei Lugovoy, the former KGB colonel, who not only met Mr Litvinenko on the day he appears to have been poisoned but also allegedly occupied a hotel room where traces of polonium-210 have been found. Mr Lugovoy has told The Times that he has nothing to hide. Even so, he has been unavailable since the Scotland Yard team’s arrival: they have been denied access to him at a clinic where a third figure in the affair is said to be suffering from acute radiation sickness.

It would be wrong to take entirely at face value Mr Litvinenko’s self-assessment as a persecuted crusader for justice. His loyalties and business dealings were complex and possibly compromised. That he was a strange man does not make his murder any less sinister. The Kremlin had at least three compelling reasons to wish to silence him. First, he claimed before his death to have evidence linking the murder of Anna Politkovskaya, the journalist and outspoken critic of Russian policy in the Caucasus, to state security forces. Secondly, he had written a book accusing the FSB of planning to blow up an apartment building to bolster President Putin’s case for invading Chechnya in 1999. A new and heavily annotated edition of the book is due to be published next month. Thirdly, as we report today, he claimed to have uncovered a Kremlin-backed plan to blackmail or eliminate foreign-based Russian citizens holding assets salvaged from Yukos, the oil company founded by the jailed tycoon Mikhail Khodorkovsky.

Suggestions that the Russian state was involved in Mr Litvinenko’s murder have been dismissed by the Kremlin as preposterous. In a civilised world, they would be just that, and the murder may yet prove to be the result of a private business deal that went wrong. A more likely scenario, however, is that Mr Litvinenko was the victim of over-mighty, underemployed Russian security forces that are themselves increasingly abusing their power in business dealings. The rise of the FSB to the dominant position that its predecessor, the KGB, once enjoyed, fuels corruption, inhibits the economy and democracy, and has the potential to become a serious political embarrassment for Mr Putin. Yet it is largely a problem of his own making, as is the rise of the slavishly pro-Putin youth groups accused of harassing Britain’s Ambassador to Moscow for attending an opposition conference last summer. Mr Putin should remember that power corrupts, and centralised power corrupts the figure at the centre.