Although most foreigners probably assumed that last night's explosion was caused by a bomb, that was not the natural assumption of many Muscovites.

They had another suspect in mind: Gas.

Gas leaks in old apartments are a frequent cause of accidents in Russia and are a regular feature of television news broadcasts.

But the sheer scale of the blast and the number of people signalled that this was probably caused by something else and the security services are now suggesting that it was indeed a bomb.

The mayor of Moscow, Yury Luzhkov, said it was possibly connected to a bomb explosion last week in Manezh Square just outside the Kremlin in which one woman died and many people were injured.

If this was a bomb, that still leaves a number of possible suspects.

The possible suspects

The initial cause of the blast was thought to be a gas leak

Criminal gangs in Russia are increasingly well-armed and ruthless and have used car-bombs to blow up their enemies for a number of years.

So it is possible that someone in this block of flats was the victim of a mafia assassination.

Maverick political extremist groups have also been suspected of staging a series of acts of terror recently that include blowing up a statue of the last tsar Nicholas II and the still unsolved murder of the popular liberal politician Galina Starovoitova.

The Dagestan connection

But most people are making a connection with the events in Dagestan, where Islamist guerrillas are fighting Russian forces.

There is supporting evidence for that from both the Russian news agency Interfax and the German radio station Deutsche Welle, who have both reported that a man with a Caucasian accent had telephoned claiming responsibility.

There are several thousand militants currently fighting the Russians in the southern republic of Dagestan.

They are not only opposed to everything Moscow stands for, but also to official Chechen President Aslan Maskhadov.

Over the last few weeks they have gained additional motives for revenge as Russian planes have bombed their villages in Dagestan and Chechnya and killed their comrades.

That makes a North Caucasian militant, either a freelancer or someone working directly for the militants the most likely suspect at the moment.

And yet many questions still remain unanswered: Why was a bomb planted in an ordinary Moscow apartment block? What did a terrorist hope to achieve apart from sowing panic in the Russian capital?