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Early indications are that the bomber was a 30-year-old woman from Russia's semiautonomous republic of Dagestan, Russia's Investigative Committee said.
Video taken behind the bus from another vehicle -- aired by state-run TV outlet Russia 24 -- shows the bus moving on a busy road when the explosion occurred around 2 p.m., with at least one vehicle swerving out of the way.
Of the 33 injured, eight were in critical condition Monday evening, the Investigative Committee said.
Investigators say they think the bomber boarded the bus at a stop, and the blast happened shortly afterward, the committee said, citing in part a woman who survived the blast.
Dagestan is part of Russia's North Caucasus region, where Islamic militants have been fighting Moscow's rule. Volgograd is more than 400 kilometers (250 miles) north-northwest of Dagestan's northern boundary.