As soon as news emerged of the explosion in the Moscow subway, suspicion immediately fell on Chechen separatists.

The Chechens were blamed for a series of bomb blasts which destroyed blocks of flats in Russia last September.

But doubts remain as to who was responsible, and this explosion may re-open that debate.

The threat of bomb explosions was unknown in Moscow in Soviet times.

Gangland wars

But in recent years it has become a danger of which Muscovites are well aware.

Gangland wars, which have accompanied the growth of Russia's raw capitalism, have seen shootings and bombings, notably one on the 10th floor of the Intourist Hotel.

An increase in anti-Semitism has seen synagogues made the targets of bomb attacks.

But it was the bombings last year which caused the greatest uproar.

In June, a small device exploded in the plush underground Manezh shopping complex.

Professionalism

Then in September, two blocks of flats in Moscow, and one in the southern town of Volgodonsk, were destroyed by huge explosions.

These incidents were immediately blamed on Chechen separatists, and were used to justify the entry of Russian troops into the breakaway republic later that month.

But the scale of those attacks, and the professionalism with which they were carried out, led to doubts as to whether the Chechens could have arranged such acts.

Threat

And although a number of arrests have been made, no-one has been brought to justice for them.

The Russians have done much better in this campaign in Chechnya than they did in the first war, between 1994 and 1996.

But they have certainly not eliminated the separatists, and, unless proved otherwise, many people will believe that the Chechens have now realised their long-standing threat to carry their struggle for independence to the Russian capital.