Umarov, who had claimed responsibility for the 2010 and 2011 bombings, ordered a halt to attacks on civilian targets during the mass street protests against President Vladimir Putin in the winter of 2011-12. He reversed that order in July, urging his men to "do their utmost to derail" the Sochi Olympics which he described as "satanic dances on the bones of our ancestors."
A group calling itself Anonymous Caucasus said in a statement Friday on the Caucasus rebel web site, kavkazcenter.com, that it would launch cyber-attacks to avenge Russia's refusal to acknowledge the 19th-century expulsion of Chirkassians, one of the ethnic groups in the Caucasus.
The International Olympics Committee expressed its condolences over the bombing, but said it was confident of Russia's security preparation for the games.
"At the Olympics, security is the responsibility of the local authorities, and we have no doubt that the Russian authorities will be up to the task," it said in a statement.
Russian authorities have introduced some of the most extensive identity checks and sweeping security measures ever seen at an international sports event.
Anyone wanting to attend the games that open on Feb. 7 will have to buy a ticket online from the organizers and obtain a "spectator pass" for access. Doing so will require providing passport details and contacts that will allow the authorities to screen all visitors and check their identities upon arrival.
The security zone created around Sochi stretches approximately 100 kilometers (60 miles) along the Black Sea coast and up to 40 kilometers (25 miles) inland. Russian forces include special troops to patrol the forested mountains towering over the resort, drones to keep constant watch over Olympic facilities and speed boats to patrol the coast.
The security plan includes a ban on cars from outside the zone from a month before the games begin until a month after they end.
In Washington, the State Department condemned the bombing and said the U.S. stands "in solidarity with the Russian people."