A bomb has gone off inside a bus in the central Russian city of Volgograd killing six people, according to Interfax News Agency.
The blast also wounded 20 people, according to reports. Police have opened a criminal case for murder.
"According to preliminary data, an unknown explosive device went off inside the bus, that's why a terrorist act may be viewed as one of the possible causes of the blast," a source in the Investigative Committee told Interfax.
The source said detectives suspect a female suicide bomber to be responsible for the blast.
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