A female suicide bomber attacked a bus in southern Russia on Monday, authorities said, killing at least six people in the deadliest such blast outside the volatile North Caucasus region in nearly three years.
The bombing in Volgograd was likely to raise fears of further attacks by Islamist militants as Russia prepares to host the 2014 Winter Olympics in February in the Black Sea resort city of Sochi, not far from the mainly Muslim North Caucasus.
The attack, which investigators blamed on a 30-year-old woman from Dagestan - the North Caucasus province at the centre of an insurgency - also wounded 28 people, of whom eight were in critical condition, the federal Investigative Committee said.
State television showed footage, taken from a camera mounted on a driver's dashboard, of an explosion ripping through the bus as it travelled along a tree-lined road, sending shards of metal and glass flying.
Passengers scrambled out of doors and windows as the bus came to a stop engulfed in a cloud of smoke.
"There was a blast - a bang - all the glass flew out of the windows," an eyewitness named Ivan, who had been driving behind the bus, told state-run Rossiya-24 television.
"The cloud of smoke quickly dissipated and then I saw people start to fall out and run out to escape the bus," he said. "It was a horrible sight."
Authorities named the suspect as Naida Asiyalova, 30, and state TV showed a passport picture of her in a black chador.
"This woman, in a hijab, got on the bus at one of the stops and the explosion occurred almost immediately afterwards," Investigative Committee spokesman Vladimir Markin said.
A law enforcement source in Dagestan told Reuters that she had been the wife of Dmitry Sokolov, a man from the Moscow suburbs who joined an insurgent group in Dagestan last year.
The two met online, the police source said. Asiyalova then moved to Moscow to marry Sokolov, 20, 10 years her junior. In July 2012, his parents put out a missing persons alert for him when he failed to come home from Arabic classes.
The source described Sokolov as an explosives expert, who is thought to have prepared a suicide belt used by a woman who blew herself up near federal police headquarters in Dagestan's capital Makhachkala in May, killing two people.
"By all appearances, he prepared Naida Asiyalova for her suicide bombing," the police source said.
Vladimir, a man who said his daughter survived the bombing, said many students were on the bus. "The blast was big, it was huge," he told Ekho Moskvy radio. "When I came to pick her up, half the bus was simply not there. It was scary. Very scary."
There was no immediate claim of responsibility.
Volgograd is a city of around one million people that lies 900 kilometres southeast of Moscow and a few hundred kilometres north of the North Caucasus and Sochi, at the western end of the Caucasus range, where Russia will hold the Winter Olympics.
President Vladimir Putin has staked his reputation on the Games and ordered authorities to boost security in the North Caucasus, where the insurgency is rooted in two post-Soviet wars pitting Chechen separatists against the Kremlin.
Putin's spokesman conveyed his condolences to the wounded and relatives of the dead, but Putin made no public comment.
Insurgents who say they are fighting to create an Islamic state have claimed responsibility for a suicide bombing that killed 37 people at a Moscow airport in 2011 and twin suicide bombings that killed 40 people on the Moscow subway in 2010.
The latter attack was carried out female suicide bombers, dubbed "black widows" in Russia because their male relatives have often been killed by security forces.
In 2002, Chechen women wearing black chadors and suicide belts also took part in a three-day Moscow theatre hostage siege in which around 130 people were killed.