MOSCOW (AP) -- Russian investigators were trying Thursday to determine who killed six men whose bodies were found in four cars abandoned in an area of southern Russia close to the volatile Caucasus Mountains.

Three of the cars had been rigged with explosive devices, but only one of the bombs went off and no one was hurt. The victims had been shot, according to investigators.

The killings, discovered Wednesday on the outskirts of Pyatigorsk, further heightened security concerns ahead of the Winter Olympics in Sochi. Both cities lie near the Caucasus region, where an Islamic insurgency is simmering.

Vladimir Markin, spokesman for Russia's main investigative agency, said in a statement that Federal Security Service officers had joined the investigation, and that no motive had yet been determined.

NTV television reported from the scene that security had been heightened on the nearby border with Kabardino-Balkaria, one of several predominantly Muslim republics in Russia's Caucasus.

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.


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