MOSCOW — A suicide bomber struck a busy railway station in southern Russia on Sunday, killing at least 15 other people and wounding scores more, officials said, in a stark reminder of the threat Russia is facing as it prepares to host February’s Olympics in Sochi.

No one immediately claimed responsibility for the bombing in Volgograd, but it came several months after Chechen rebel leader Doku Umarov called for new attacks against civilian targets in Russia, including the Sochi Games.

Suicide bombings have rocked Russia for years, but many have been contained to the North Caucasus, the center of an insurgency seeking an Islamist state in the region. Until recently Volgograd was not a typical target, but the city formerly known as Stalingrad now has been struck twice in two months – suggesting militants may be using the transportation hub as a renewed way of showing their reach outside their restive region.

Volgograd, which lies close to volatile Caucasus provinces, is 550 miles south of Moscow and about 400 miles northeast of Sochi, a Black Sea resort flanked by the North Caucasus Mountains.

The bombing highlights the daunting security challenge Russia will face in fulfilling its pledge to make the Sochi Games the “safest Olympics in history.” The government has deployed tens of thousands of soldiers, police and other security personnel to protect the games.

Through the day, officials issued conflicting statements on casualties. They also said that the suspected bomber was a woman, but then reversed themselves.

The Interfax news agency quoted unidentified law enforcement agents as saying that footage taken by surveillance cameras indicated that the bomber was a man. It also reported that it was further proven by a torn male finger ringed by a safety pin removed from a hand grenade, which was found on the site of the explosion.

The bomber detonated explosives in front of a metal detector just beyond the station’s main entrance when a police sergeant became suspicious and rushed forward to check ID, officials said. The officer was killed by the blast, and several other officers were wounded.

“When the suicide bomber saw a policeman near a metal detector, she became nervous and set off her explosive device,” Vladimir Markin, the spokesman for the nation’s top investigative agency, said. He added that the bomb contained about 22 pounds of TNT and was rigged with shrapnel.