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.
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Investigators say they think a woman from Dagestan set off the blast
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The explosion happened on the bus around 2 p.m. Monday, authorities say
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A criminal investigation is under way, authorities say
Volgograd is more than 400 kilometers (250 miles) north-northwest of @placeholder's northern boundary.