MOSCOW — An explosion at a railroad station in central Russia has killed at least 13 people, according to initial reports, raising the specter of a wave of terrorism ahead of the Winter Olympics in Sochi. Dozens of others were wounded, meaning the death toll could still rise.

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The explosion, which officials said was caused by a bomb, struck the main railroad station in Volgograd, a city about 550 miles south of Moscow, at 12:45 p.m. The blast occurred near the metal detectors that have become a common security fixture at most of Russia's transportation hubs, suggesting that an attack inside the building or aboard a train might have been thwarted.

If proved to be a terrorist act, as officials initially suspected, it would be the second in Volgograd in barely two months. In October, a woman identified as Naida Asiyalova detonated a vest of explosives aboard a bus in the city, killing herself and six others.

The authorities linked her by marriage to an explosives expert working with an Islamic rebel group in Dagestan, the southern republic where the police have struggled to suppress an insurgency by Muslim separatists. A month later, the authorities announced that they had killed her husband and four others in a raid in the region.

The republics of the North Caucasus – from Dagestan to Chechnya to Adygea – have for nearly two decades been embroiled in complex, ever-shifting armed conflicts that the International Crisis Group recently called "the most violent in Europe today."

The violence has left hundreds dead already this year, and prompted the authorities to make extraordinary efforts to keep it from reaching Sochi, the Black Sea resort city that will be the host of the Winter Olympics six weeks from now.