Russian investigators are trying 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 were rigged with explosive devices, but only one of the bombs went off and no one was hurt.

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.

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

The cars with the six dead men were discovered Wednesday on the outskirts of the city of Pyatigorsk.