A suspected female suicide bomber set off a blast on a bus packed with students in southern Russia yesterday, killing six people and raising security fears less than four months before the Winter Olympic Games.
Officials said the explosion that injured nearly 30 people went off at about 2pm local time in the Volga River city of Volgograd, about 900 kilometres southeast of Moscow.
Footage on state television showed a green and white city bus mangled in the middle of the street, its windows blown out on the left side. "It was an unidentified explosive device," a spokesman for the National Anti-Terror Committee said.
Video: Driver records 'Black widow' suicide bus bombing
The Investigative Committee, Russia's equivalent to the FBI, said officials had opened a formal terror probe.
"According to preliminary information, a native of Dagestan, 30-year-old Naida Asiyalova, blew herself up," investigators said in a statement, referring to one of the North Caucasus' most violent regions.
"She boarded the bus at one of the bus stops and an explosion took place almost immediately afterwards."
An official at the Investigative Committee said the suspected bomber was the wife of a rebel commander.
"She had recently adopted Islam," the unnamed Investigative Committee source told Russian news agencies.
Officials said the packed bus was carrying about 40 passengers at the time of the explosion.
"There were lots of students on the bus," said a man whose daughter survived the explosion. "It was a powerful explosion - a huge blast.
"The bus was torn to pieces. When I came to pick up [my daughter], half the bus was simply not there."
A regional emergencies ministry spokeswoman said 17 people were hurt in the explosion, although some Russian news agency reports put the number of injured as high as 27.
Initial reports had also put the death toll at six, although most local and national security agencies had later revised the number of dead down to five.
"All those who need help are getting full assistance," an unnamed Volgograd government official separately told the Interfax news agency.
"We have gathered the required amount of medicine, and called up additional doctors," the official said.
Security remains a concern throughout southern Russia ahead of the February 7 to 23 Winter Olympic Games in the Black Sea resort of Sochi.
Russia fought two wars in the post-Soviet era in the volatile North Caucasus republic of Chechnya, and has since seen violence spill over to neighbouring Muslim regions such as Ingushetia and Dagestan.
Female suicide bombers are often referred to in Russia as "black widows" - women who seek to avenge the deaths of their family members in North Caucasus fighting by targeting Russian civilians.
Two female suicide bombers set off blasts at two Moscow metro stations in March 2010 that killed more than 35 people. So-called black widows were also responsible for taking down two passenger jets that took off from a Moscow airport within minutes of each other in 2004, killing about 90 people.
Several also participated in the September 2004 Beslan school hostage crisis that ended with the death of at least 335 people.