Police in Moscow are searching for clues in the apparent suicide bombing of a crowded subway train. Thirty nine people were killed in the blast.
Investigators are searching the wreckage of the subway car that was destroyed in Friday's blast, looking for evidence of what kind of bomb caused the extensive devastation.
Other police have questioned some of the scores of people now being treated in city hospitals. A surveillance videotape appears to show two people who boarded the train just before the bomb went off.
Security has been tightened at all 170 subway stations around Moscow, where people are jittery about the possibility of another attack.
The added police presence has failed to stem criticism in the local media about what one prominent newspaper calls the failure of the Federal Security Service, or FSB, to prevent Friday's attack.
Similar criticism followed the deadly hostage-taking siege by Chechen militants at a Moscow theater in October 2002, in which 130 hostages died when Russian troops finally stormed the theater after a three-day standoff.
On Friday President Vladimir Putin blamed the subway attack on the Chechens, as well, saying the powerful explosion was clearly an act of terrorism.
Most officials agree with that assessment, although police say final word will only come once the investigation is completed.
Some human rights groups and politicians have criticized Mr. Putin for making his accusation before all the facts are known.
In an appearance on television, Mr. Putin even blamed the latest attack on Chechen leader Aslan Maskhadov, who denied any involvement.
The most notorious Chechen rebel commander, Shamil Basayev, has claimed responsibility for previous attacks, including the theater siege.
The Russian president has long rejected calls to enter into some kind of dialogue with the separatists, who have been fighting Russian troops in the breakaway region for almost a decade. On Friday, he said "We don't negotiate with terrorists, we eliminate them."
Over the past year, the Kremlin has sought to show the war in Chechnya is all but over, despite daily fighting in the rebel region, as well as attacks in Moscow and elsewhere.
While the president's popularity remains high with the Russian public, polls also show about half the country thinks some kind of political settlement should be pursued.
Friday's incident came just five weeks before the presidential election, in which Mr. Putin looks set to easily win election to a second term.