A suicide bomber detonated the equivalent of over 20 pounds of TNT near the entrance to a railway station in the central Russian city of Volgograd Sunday, killing at least 15 people and injuring dozens.

The blast, which blew out the front windows of the huge Stalin-era structure, was recorded by CCTV and rebroadcast by the state-funded RT network.

In a statement the Kremlin's Investigative Committee, Russia's top police body, said that the bombing was, "according to available evidence," the work of a female suicide bomber who triggered the device, which was loaded with shrapnel, as she approached the metal detectors near the station's entrance and became nervous when she spotted a police officer. According to the statement, the casualties might have been far greater if she had succeeded in penetrating into the inner waiting area, which was crammed with New Year's travelers preparing to board trains.

No one has claimed responsibility.

A similar bombing barely two months ago, which demolished a Volgograd city bus and killed six people, was revealed to be the work of a female suicide bomber from Russia's insurgency-racked southern province of Dagestan. Such women have been dubbed "black widows" because they often turn out to be family members of Islamist rebels killed by Russian security forces, recruited to stage revenge attacks on "soft" Russian targets.

Similar attacks have killed thousands in several Russian cities over the past decade-and-a-half, but the approaching Sochi Winter Olympics has likely stimulated terrorist planners such as Chechen Islamist warlord Doku Umarov to step up their activities.

Russian media reports suggest that Sochi itself, garrisoned with around 40,000 special police and protected by an array of high-tech security measures, as well as the capital city of Moscow, may have been made relatively impregnable to terrorist infiltration. But scores of other large Russian cities, such as Volgograd, have received less attention and clearly remain vulnerable.

The bombing is bound to increase anxieties in the Kremlin, with the opening of the Sochi Games barely a month away and President Vladimir Putin's prestige heavily invested in a successful outcome. Last week he ordered a sweeping prison amnesty that effectively removed some of the deepest human rights controversies between Moscow and the West by freeing prisoners such as former oil tycoon Mikhail Khodorkovsky, two members of the band Pussy Riot, and 30 international Greenpeace activists who were being held for protesting at an Arctic drilling rig.

Mr. Putin's spokesman, Dmitry Peskov, told journalists Sunday that the president is actively overseeing the investigation into the Volgograd bombing. "Vladimir Putin instructed the ministers and the heads of law enforcement departments to take all necessary measures to establish the causes and circumstances of the incident, identify and bring to justice those who are behind it," Mr. Peskov said.

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Andrei Soldatov, editor of Agentura.ru, an online journal that studies the security services, warns that the two Volgograd attacks in recent months demonstrate that terrorists from the turbulent North Caucasus have the capability to strike repeatedly at major Russian targets.

"The real fear is that these Volgograd bombings could be diversionary attacks," he says. "This has happened before; we know the terrorists use such tactics. If they are planning something big, attacks like this can distract the security forces, make them divert resources from the main target, and generally sow uncertainty everywhere.... They clearly have the ability to strike beyond the North Caucasus region, and the people to carry it out. There is no reason to assume that Sochi and Moscow are safe, just because they're hitting Volgograd," he says.