Russia's Prime Minister Vladimir Putin said on Wednesday the blasts in Moscow and Dagestan that killed 51 people this week may be the work of the same criminal group.
"I do not rule out that the same gang was at work here," a stern-looking Putin told a government meeting in televised remarks.
He said it did not matter where the bombings took place or whether victims of those bombings were Orthodox Christians, Muslims or people of other faith groups.
"This is a crime against Russia," Putin said. "The special services are at work," he added.
Attacks on officials and security personnel have been almost a daily occurrence in Russia's Northern Caucasus and usually generate little attention in Moscow.
By contrast, the metro bombings in Russia's heartland have shaken Russians to the core after a lull of six years in militant attacks.
Islamic militants pledged last month to bring a holy war to Russia's regions.
Observers have said the Moscow attacks exposed Russia's vulnerability in the face of suicide bombings, adding that the special services had not learnt from the past suicide bombings that shook the country in early 90s.
On Tuesday, Putin said the bombing masterminds should be scraped out from the sewers, his language resembling his famous pledge in 1999 to strike at rebels even in the "outhouse."
On Monday, two female suicide bombers blew themselves up on the packed Moscow metro, killing 39 people. On Wednesday, two consecutive explosions in the southern region of Dagestan killed another 12 people, nine of them police.