At least 38 people were killed in twin explosions at stations on Moscow's metro rail network [EPA]
Twin explosions at two stations on Moscow's metro rail network have left a trail of death and destruction.
Al Jazeera talks to three experts on Russia about who they think is behind Monday's bombings.
|Martin McCauley, Russian expert, University of London
"I think it's 90 per cent certain [the attacks came from the Caucasus], but in Russia nothing is certain.
There are other opponents of the regime, we had a train derailed between Moscow and St Petersburg and it's very difficult to say who exactly was responsible.
So it's possible somebody else was responsible, but I think the balance of responsibility is that this came from the Caucasus.
For instance, one of the explosions was outside the Lubyanka station, just beside the headquarters of the FSB [Russian security services], the old KGB.
And the FSB are deeply involved in the North Caucasus, in Chechnya and so on, and the Chechens and others deeply resent their presence there, and the crimes they have committed, they claim.
So, therefore, you could see this as a revenge attack. It's also Holy Week in Russia, that's another factor.
And from my point of view, I'm really surprised it's taken so long for these bombings to occur.
There have been so many car bombings and things like that in the North Caucasus over the last few months, one woud have expected them to head for Moscow, and also St Petersburg, which has a first-class metro system.
The security situation [in the Caucasus] is getting even worse, because previously, if you go back 10, 15 years, the violence was really concentrated in Chechnya.
Now it's spread into neighbouring republics, Dagestan, which was previously very, very peaceful, Ingushetia, Kabardino-Balkariya and so on.
It has spread throughout the North Caucasus and the FSB think there's about 200 activists who are really a problem for them, and they're targeting them.
But the trouble is they're using such brutal methods that they antagonise the local population."
|Alexander Nekrassov, Russia analyst and former adviser to the Kremlin
"I think that probably suspicions centre on the rebels operating in the North Caucasus, primarily in Chechnya and Ingushetia.
Especially the fact that women suicide bombers were involved, that is usually the signature of these groups.
The fact is by targeting Lubyanka, which is the station beneath the FSB, former KGB, building, basically tells us that this is a challenge to the Kremlin, to the government, saying 'we can reach you, we can get to you at any moment'.
The situation in Ingushetia has deteriorated gradually and has turned into a sort of civil war simmering there.
In Chechnya, according to my information, the situation has been also tense for the last year at least, and especially in the last several months.
In a sense, some people were expecting some terrorist attack to take place, nobody obviously was expecting something like that, just generally thinking that something might happen.
And I heard that security was actually tightened quietly in Moscow, but unfortunately it's very difficult to control every place, this is a soft target again."
|Marsha Lipman, political analyst at the Carnegie Moscow Centre
"It's been six years since the last terrorist attacks in Moscow.
The year 2004 was really horrible, with several major terrorist attacks in Russia, the worst being Beslan, where terrorists took a whole school hostage and over 300 people were killed.
Since then we have not had any major terrorist attacks outside the North Caucaus.
Russia has a tragic history of terrorism originating in the North Caucasus.
Early on in the 1990s the cause was secession from Russia, now it's hard to tell.
North Caucasus republics, at least several of them, have become scenes of all sorts of strife, all sorts of feuds and violence.
So it's really hard to say who is behind it and what these people seek."