Dmitry Astakhov / AP
Russian authorities said on Thursday that security forces had been put on combat alert in the southern Stavropol region after the discovery of six bodies with gunshot wounds in four different cars, three of which were rigged with explosives.
Only one of the bombs went off and no one was hurt. But the killings are further heightening security concerns ahead of the Winter Olympics in Sochi, which also lies near the Caucasus region, where an Islamic insurgency is simmering.
The corpses were discovered on Wednesday in two separate districts outside the regional capital Stavropol, a gateway to the North Caucasus, where Russia faces an insurgency by Islamist militants who have threatened to try to prevent the Olympics going ahead.
Vladimir Markin, spokesman for Russia's main investigative agency, said in a statement issued on Thursday that Federal Security Service officers had joined the investigation. He said no motive had yet been determined.
Mr Putin said after the Volgograd attacks that he would annihilate all "terrorists" in Russia.
The Winter Olympics open in Sochi on Feb. 7. The Black Sea resort is on the western edge of the Caucasus mountains where the insurgents want to carve out an Islamic state.
The head of Russia's Olympic Committee has said no more can be done to safeguard the Games because every measure possible is already in place.
Russian forces went on combat alert in Sochi on Tuesday and about 37,000 personnel are now in place to provide security at the Games, Russian officials say.
Edited by Hannah Strange
