Two weeks before the Winter Olympics, Russian security forces are reportedly searching for potential suicide bombers, at least one of whom may already be in the host city of Sochi.
The suspects are thought to be linked to Islamist militants who are fighting to throw off Russian control and create a fundamentalist Muslim state in Russia's North Caucasus Mountains.
Police have been circulating leaflets at hotels in Sochi, warning about women who may be part of a terrorist plot.
They are known as "black widows," women sent to carry out suicide bombings in revenge for husbands or family members killed by security forces.
It's a tactic that's been used before, to devastating effect, by a Chechen rebel leader named Doku Umarov.
Last June, Umarov released a video that showed him in a forest, flanked by jihadi fighters. He called on Islamist militants to do everything in their power to wreck the Olympics, which he called "satanic dances on the bones of our ancestors."
Umarov has claimed responsibility for a number of deadly suicide attacks in the past, including bombings in Moscow in 2010 and 2011 that killed more than 70 people.
Mark Galeotti, an expert on Russian security issues, says that Umarov is an important influence on other jihadists in the region, "but his actual authority within the insurgent movement is exceedingly limited. Essentially, it's symbolic more than anything else."
Galeotti, a professor at New York University, says that's because the insurgency is composed of autonomous cells scattered across a rugged area that stretches for hundreds of miles.
It makes the rebel network hard to combat, because each cell plans and carries out its own operations.
Earlier this week, an insurgent group based in Dagestan posted an online video showing two men in explosive vests, saying that unless Russian President Vladimir Putin canceled the Winter Games, they were preparing a "present" for him and the Olympic visitors.
The video also claims that the two men pictured were the ones who carried out a pair of suicide bombings that killed 34 people last month in the southern Russian city of Volgograd.
Although it's only a little more than 400 miles from Sochi, Volgograd is considered to be outside the North Caucasus, a reminder that the insurgents have the power to strike terror beyond their region.
Some analysts say that terror may have only been a secondary purpose of the attacks.
"The biggest fear is that these attacks in Volgograd might be some sort of a diversion tactics," says Andrei Soldatov, editor-in-chief of Agentura.ru, a website that acts as a watchdog on the Russian security services. "Before every big terrorist attacks in Russia, militants used diversions. They organized small terrorist attacks in some other regions."
It was only after the suicide attacks focused intense attention on Volgograd that reports emerged that a "black widow" might have infiltrated the security cordon around Sochi.
A leaflet that has been distributed in the Olympic city shows a mug shot of a woman with dark, impassive eyes.
She wears a Muslim headscarf.
The leaflet says she is Ruzanna Ibragimova, the 22-year-old widow of an insurgent, and that she has been spotted in recent days in central Sochi.
Galeotti points out that once a suicide bomber has been prepared, he or she must be must be used fairly quickly.
"Suicide bombers are actually quite fragile weapons," he says. "These people have been groomed, they have been brought to a pitch, where they're ready to give their life. And once they're at that pitch, you can't then put them on the shelf until you're ready."
Even if there is no bomber, and even if the Olympics go off without a hitch, the terrorists may have already succeeded, to some degree, in disrupting the games.
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