Monday, September 13, 1999 Published at 11:43 GMT 12:43 UK
Analysis: The likely suspects
After the fourth explosion, not many people believe it was an accident
After the fourth explosion in Moscow within a fortnight, most Muscovites are convinced that all four were caused by bombs.
But who planted them? There is more than one group that may have a motive as well as the means to carry out such attacks.
The Dagestan connection
Most Russians are making a connection with the events in Dagestan, where Islamist guerrillas are fighting Russian government forces.
Several thousand militants are currently fighting the Russian army in the southern republic of Dagestan.
Most entered Dagestan from the neighbouring republic of Chechnya, which has been beyond Russia's control for the past three years.
The mayor of Moscow, Yuri Luzhkov, has said he is confident the explosions are the work of forces originating from Chechnya, rather than Dagestan itself.
That makes a North Caucasian militant, either a freelancer or someone working directly for the militants, the most likely suspect at the moment.
However, none of the militant leaders has yet claimed responsibility. The leading Chechen warlord, Shamil Basayev, told a news agency that neither he nor his Saudi-born ally, Khattab, had anything to do with the explosions.
Muslims from the North Caucasus are often regarded by Muscovites as potential criminals and troublemakers.
President Boris Yeltsin has warned that the explosions could have been caused by forces hoping to destabilise the government by causing inter-ethnic tensions.
"One must not shift the problem onto an ethnic or religious basis. It is impermissible to carry out purges along ethnic lines," he said.
Although most people still blame Islamic guerillas, there are other groups in Russia who have a history of violence.
It is possible that someone in each of the targetted buildings was the victim of a mafia assassination, though this is not thought to be a likely explanation.
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.
But many questions still remain unanswered: Why were bombs planted in ordinary Moscow apartment blocks? What did a terrorist hope to achieve apart from sowing panic in the Russian capital?
A conclusive verdict on the tragedies may take weeks to emerge - or may never come to light at all.