Russia's security service says it still has no leads in the Moscow subway bombing on Friday that killed 39 people and wounded more than 100.
Security has been increased at all Metro stations in Moscow, as well as on all highways leading into Russia's capital city.
Moscow mayor Yuri Luzhkov announced that registration procedures for all people traveling into the capital will be "powerfully strengthened" and directed at foreigners as well as people from Chechnya and the Caucasus Mountain region.
Within hours of the rush-hour subway explosion Friday morning, President Vladimir Putin said he was certain it was the work of what he called "bandits and terrorists" from Chechnya. Similar bombing incidents in recent years have been admitted by Chechens.
However, some investigators picking through the wreckage of the destroyed subway car say they have not ruled out the possibility that the explosion might have been accidental.
Spokesman Sergei Ignatchenko of the Federal Security Service says there are no firm leads on suspects in the case. He says some form of explosive material might have gone off accidentally while being transported on the train.
No one has claimed responsibility for the attack.
In his televised remarks Friday President Putin singled out Chechen leader Aslan Maskhadov for the bombing. But a spokesman for Mr. Maskhadov denied the charge and denounced the subway attack as an act of terrorism.
Mr. Maskhadov was elected president of Chechnya in 1997 but has been in hiding, probably somewhere in the mountains of Chechnya, since Mr. Putin launched the latest war there two years later.
The Chechen leader has also never exercised control over all Chechen rebel commanders, especially the most notorious warlord, Shamil Basayev. Mr. Basayev has claimed responsibility for previous attacks including several suicide bombings.
Monday has been declared a day of mourning for the victims of the bombing.