Russian authorities are on high alert Thursday after finding six dead men in four abandoned cars, three of which had been rigged with explosives.

Police said all the victims had been shot. The bodies were discovered Wednesday near the town of Pyatigorsk, The Guardian reported.

Only one of the bombs actually detonated, and nobody was injured in the blast, investigators told The Guardian.

Investigators said the Federal Security Service is investigating but hasn’t yet determined the motive.

The finding set much of the country on edge. Russia had already ratcheted up security ahead of next month’s Winter Olympics in Sochi, amid suicide bombings that killed 34 and injured dozens more in separate attacks in Volgograd in December.

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