A suicide bomber has struck a busy railway station in southern Russia, killing at least 16 people and wounding scores more, officials have reported, in a stark reminder of the threat Russia is facing as it prepares to host February's Olympics in Sochi.
Nobody 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 centre 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.
The city lies about 900km south of Moscow and about 650km north-east of Sochi.
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, and police to protect the Games.
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, spokesman for the nation's investigative agency, said. He added that the bomb contained about 10kg of TNT and was rigged with shrapnel.
Mr Markin said that another hand grenade, which didn't explode, was also found on the explosion site.
He added 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.
Mr Markin said 13 people and the bomber were killed on the spot, and the regional government said two other people later died in hospital. About 40 were admitted to hospital, many in grave condition.
Earlier in the day, Lifenews.ru, a Russian news portal that reportedly has close links to security agencies, posted what it claimed was an image of the severed head of the female attacker. It 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 Volgograd bus, killing six people and injuring about 30. Officials said the attacker came from Dagestan, which has become the centre of the Islamist insurgency that has spread across the region after two separatist wars in Chechnya.
As in yesterday's blast, her bomb was rigged with shrapnel that caused severe injuries.
Chechnya has become more stable under the grip of Moscow-backed strongman Ramzan Kadyrov, who incorporated many former rebels into his feared security force. But in Dagestan, set 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 officials.
Rights groups say that authorities' tough response involving arbitrary arrests, torture and killings of terror suspects has fuelled 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 yesterday's moment of explosion, a bright orange flash inside the station behind the massive main gate followed by plumes of smoke.
"As soon as I walked up to the station entrance, all hell broke loose. All the doors, windows scattered," one witness said.
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."
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 people.
Following yesterday's explosion, the Interior Ministry ordered police to beef up patrols at railway stations and other transport facilities across Russia.
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".
The International Olympics Committee said it was confident of Russia's security preparation for the Games.
The security zone created around Sochi stretches about 100km along the Black Sea coast and up to 40km inland. Russian forces include special troops to patrol the forested mountains towering over the resort, drones to keep constant watch over Olympic facilities and boats to patrol the coast.