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 has now 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 and said the attacker could have been a man.

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 policemen 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 in a statement earlier in the day. He added that the bomb contained about 10 kilograms (22 pounds) of TNT and was rigged with shrapnel.

Markin later told Interfax that the attacker could have been a man, but added that the investigation was still ongoing. He said that another hand grenade, which didn’t explode, was also found on the explosion site.

Markin argued that security controls prevented a far greater number of casualties at the station, which was packed with people at a time when several trains were delayed. About 40 were hospitalized, many in grave condition.

Earlier, Lifenews.ru, a Russian news portal that reportedly has close links to security agencies, even posted what it claimed was an image of the severed head of the female’s attacker. It even said the attacker appeared to have been a woman whose two successive rebel husbands had been killed by Russian security forces in the Caucasus.

Female suicide bombers, many of whom were widows or sisters of rebels, have mounted numerous attacks in Russia. They often have been referred to as “black widows.”

In October, a female suicide bomber blew herself up on a city bus in Volgograd, killing six people and injuring about 30. Officials said that attacker came from the province of Dagestan, which has become the center of the Islamist insurgency that has spread across the region after two separatist wars in Chechnya.

As in Sunday’s blast, her bomb was rigged with shrapnel that caused severe injuries.

Chechnya has become more stable under the grip of its Moscow-backed strongman, who incorporated many of the former rebels into his feared security force. But in Dagestan, the province between Chechnya and the Caspian Sea, Islamic insurgents who declared an intention to carve out an Islamic state in the region mount near daily attacks on police and other officials.

Rights groups say that authorities’ tough response involving arbitrary arrests, torture and killings of terror suspects has fueled the rebellion.

The Kremlin replaced Dagestan’s provincial chief earlier this year, and the new leader abandoned his predecessor’s attempts at reconciliation and his efforts to persuade some of the rebels to surrender in exchange for amnesty.

Security camera images broadcast by Rossiya 24 television showed Sunday’s moment of explosion, a bright orange flash inside the station behind the massive main gate followed by plumes of smoke.

“All the doors, windows scattered. I got a concussion and smoke billowed from inside.

Another witness, Roman Lobachev, told Rossiya television that he was putting his bags on a belt for screening when he heard the sound of an explosion. “I heard a bang and felt as if something hit me on the head,” said Lobachev who survived the attack with minor injuries.

The bombing followed Friday’s explosion in the city of Pyatigorsk in southern Russian, where a car rigged with explosives blew up on a street, killing three.

Following Sunday’s explosion, the Interior Ministry ordered police to beef up patrols at railway stations and other transport facilities across Russia.

Russia in past years has seen a series of terror attacks on buses, trains and airplanes, some carried out by suicide bombers.

Twin bombings on the Moscow subway in March 2010 by female suicide bombers killed 40 people and wounded more than 120. In January 2011, a male suicide bomber struck Moscow’s Domodedovo Airport, killing 37 people and injuring more than 180.

Umarov, who had claimed responsibility for the 2010 and 2011 bombings, ordered a halt to attacks on civilian targets during the mass street protests against President Vladimir Putin in the winter of 2011-12. He reversed that order in July, urging his men to “do their utmost to derail” the Sochi Olympics which he described as “satanic dances on the bones of our ancestors.”

A group calling itself Anonymous Caucasus said in a statement Friday on the Caucasus rebel web site, kavkazcenter.com, that it would launch cyber-attacks to avenge Russia’s refusal to acknowledge the 19th-century expulsion of Chirkassians, one of the ethnic groups in the Caucasus.

The International Olympics Committee expressed its condolences over the bombing, but said it was confident of Russia’s security preparation for the games.

“At the Olympics, security is the responsibility of the local authorities, and we have no doubt that the Russian authorities will be up to the task,” it said in a statement.

Russian authorities have introduced some of the most extensive identity checks and sweeping security measures ever seen at an international sports event.

Anyone wanting to attend the games that open on Feb. 7 will have to buy a ticket online from the organizers and obtain a “spectator pass” for access. Doing so will require providing passport details and contacts that will allow the authorities to screen all visitors and check their identities upon arrival.

The security zone created around Sochi stretches approximately 60 miles along the Black Sea coast and up to 25 miles inland. Russian forces include special troops to patrol the forested mountains towering over the resort, drones to keep constant watch over Olympic facilities and speed boats to patrol the coast.

The security plan includes a ban on cars from outside the zone from a month before the games begin until a month after they end.

In Washington, the State Department condemned the bombing and said the U.S. stands “in solidarity with the Russian people.”

Associated Press writer Steve Wilson in London contributed to this report.