Moscow ( CNN ) -- Investigators say they think a female suicide bomber caused a blast that ripped through a passenger bus in the southern Russian city of Volgograd on Monday afternoon , killing at least six people and injuring 33 others , Russian authorities said . 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 . CNN 's Phil Black contributed to this report .
Police say they think a bus from Dagestan set off the explosion . The explosion happened on the bus around 2 p.m. Monday , police say . A criminal investigation is under way , authorities say .