At least five people have been killed and 17 others injured in an explosion on a bus in the southern Russian city of Volgograd, emergency officials say.
The blast was caused by “an unspecified explosive device,” the National Anti-Terrorism Committee said in a statement.
Investigators suspected that a female suicide bomber was responsible for the bombing, the Interfax news agency reported.
Citing a source in the regional Investigative Committee office, Interfax said identity documents belonging to the suspected bomber were found near the site and that she was believed to have been the wife of a Muslim fighter.
A spokeswoman for the Emergency Situations Ministry, Irina Gogolyeva said 40 people were on the bus when the explosion occurred on Monday afternoon.
Officials had initially said investigators were probing whether the blast may have been caused by a leaking gas canister used by some city transport vehicles as a source of fuel.
Fighters who say they are fighting to create an Islamic state in Russia’s mostly Muslim North Caucasus have carried out deadly bombings inside and outside the region, made up of several provinces along Russia’s southern border.
The fighters claimed responsibility for a suicide bombing that killed 37 people at Moscow airport in January, 2011, and two nearly simultaneous suicide bombings that killed 40 people on the Moscow subway in 2010.
Volgograd is a city of around one million people that lies 900km southeast of Moscow and a few hundred kilometres north of the North Caucasus and Black Sea resort city of Sochi, where Russia will host the 2014 Winter Olympics.
President Vladimir Putin has staked his reputation on the Games and ordered authorities to boost security in the North Caucasus. [AlJazeera]