MOSCOW – Six men are dead in a series of unexplained killings involving booby-trapped bombs in southern Russia, further heightening security fears ahead of next month’s Winter Olympics in Sochi.

Investigators were scrambling Thursday to find those responsible for the six bodies found Wednesday in four abandoned cars just north of Russia’s volatile Caucasus Mountains region, where an Islamic insurgency is simmering.

Explosive devices had been placed near three of the cars, although only one of the bombs went off and no one was hurt. The victims had been shot, according to investigators.

Vladimir Markin, the spokesman for Russia’s main investigative agency, said in a statement that no motive had yet been found for the killings on the outskirts of Pyatigorsk, the center of a Russian administrative district created in 2010 to combat the insurgency. In late December, a car bomb exploded outside traffic police offices there, killing three people.

Pyatigorsk is less than 200 miles by air from Sochi, host site for the 2014 Olympics, although nearly twice as far by road.

In an indication of Russia’s unease over security ahead of the Olympics, Markin said Federal Security Service officers had joined the investigation and classified it as a counter-terrorist operation.

The shootings of local residents – at least a few of them taxi drivers – is more typical of criminal behavior, perhaps score-settling by organized gangs. But the use of explosives was suggestive of the kinds of terror attacks that take place nearly daily in the Caucasus.

Russia is still on edge following two suicide bombings in late December in Volgograd, also in southern Russia, which killed 34 people and wounded many more. No claim of responsibility has been made for those bombings, but they came several months after the leader of the Islamic insurgency called for attacks aimed at undermining the games, which run Feb. 7-23.