Police say they think a bus from Dagestan set off the explosion . The explosion happened on the bus around 2 p.m. Dagestan is part of Russia 's North Caucasus region , where Islamic militants have been fighting Moscow 's rule . Monday , police say . A criminal investigation is under way , authorities say .
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 .