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MOSCOW — Six men were fatally shot in a series of unexplained killings involving booby-trapped bombs in Russia’s North Caucasus region, further heightening security fears ahead of next month’s Olympic Games planned for the nearby resort city of Sochi.

The police found the bodies of the men Wednesday in four abandoned cars near the city of Pyatigorsk in the Stavropol region — about 170 miles east of the Olympic site — and the explosives next to the cars, a law-enforcement spokesman said. One of the devices detonated, and a bomb squad disarmed two more, he said.

No one was injured in the blast, the spokesman said, and it was not clear whether the killings were intended as an act of terrorism or were connected to gangland-style violence.

The Stavropol region borders several turbulent North Caucasus republics, where Russia is struggling to quell an Islamist insurgency that has resulted in explosions and shootouts between outlaw gangs and local police forces almost daily.

Sochi has largely been quarantined in advance of the games, closing its roads to vehicles from other parts of the country and mobilizing tens of thousands of government troops to ensure safety for athletes and tourists.

The police said they were seeking three men from the neighboring Kabardino-Balkaria region in connection with the slayings and “an attempt on the life of law-enforcement officers,” the Interfax news service reported, but did not immediately link the case to terrorism.

Vladimir Markin, the official spokesman for the Investigative Committee, the main national criminal-investigative agency in Russia, said Thursday that investigators had not determined a motive for the attack. In an indication of the unease over security ahead of the Olympics, Markin said Federal Security Service officers had joined the investigation and classified it as a counterterrorist operation.

State media, citing anonymous sources, reported that two of the slain men were taxi drivers and a third worked as a furniture assembler. There was no official identification of the victims.

The case comes as Russia is reeling from suicide bombings on a city bus and in the main railway station last month in the southern transport hub of Volgograd, which killed 34 and injured dozens more.

President Vladimir Putin vowed in a nationwide broadcast on New Year’s Eve to “continue the confident, tough and ongoing fight against the terrorists until their total elimination.”

Investigators have not identified any suspects in the Volgograd bombings, and no terrorist group has taken responsibility for the attack. Violence in the region is often linked to Doku Umarov, a terrorist leader who encouraged supporters in an online video address to use “maximum force” to disrupt the games, which he called “satanic dancing on the bones of our ancestors.”

Aleksandr Zhukov, president of Russia’s Olympic Committee, said after the Volgograd attacks that new steps to secure the Olympics would not be taken because “everything necessary has already been done.”

Material from The Associated Press is included in this report.