MOSCOW — A series of unexplained killings in southern Russia involving booby-trapped bombs has further heightened security fears ahead of next month’s Winter Olympics in Sochi.

Investigators were scrambling Thursday to determine who had killed six men whose bodies were found the day before in four cars abandoned in an area just north of the 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 Wednesday’s killings on the outskirts of Pyatigorsk, which is the center of an administrative district created in 2010 to coordinate efforts to combat the insurgency. In late December, a car bomb exploded outside the traffic police offices there, killing three people.

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

The shootings of seemingly ordinary local residents would appear to be more typical of criminal behavior, perhaps involving 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 in the run-up to February’s games.

Pyatigorsk, a city in the Stavropol region, sits just north of a string of predominantly Muslim republics in Russia’s Caucasus. NTV television reported that security had been tightened on the border on Thursday and vehicles were backed up while waiting to cross.

Sochi is located to the east of the Caucasus region on the Black Sea, less than 300 kilometers by air from Pyatigorsk, although nearly twice as far by road.

Three men whose bodies were in three of the cars have been identified: Two were taxi drivers and the third assembled furniture for a private businessman, Russian state news agencies reported, citing law enforcement agencies. Their names have not been released. The men were said to be local residents and drove inexpensive Soviet-model Lada cars.

The three other victims were found late Wednesday in a fourth vehicle. An explosive device had been placed next to the car in a metal bucket, but was defused by investigators, Markin said.

Homemade bombs also had been placed near two of the other cars; one of them went off as police approached and the other was defused.

It was unclear whether police were the target of the explosives.