At least six people died when a bus was hit by an explosion on Monday in southern Russia, the Investigative Committee said. The explosion in the city of Volgograd was reported at around 2 p.m. (1000 GMT). \"Six bodies have now been found,\" the committee\'s press service told reporters. A criminal murder case has been opened, the committee said, adding that the qualification of the case may be changed during the investigation. Twenty people were injured in the explosion, seven of whom are in critical condition, the Interior Ministry\'s main department for Volgograd region said. Local media had initially reported that four people were killed and seven others injured. The cause of the blast is yet to be determined as \"it is too early to assert that flawed gas equipment might have triggered the explosion,\" the Interfax news agency quoted an unnamed local police source as saying. The National Anti-Terror Committee said \"an unknown explosive device\" was detonated on the bus. \"Demolition experts of the Federal Security Service are examining the incident scene. An investigation team is also at work.\" Local authorities are taking measures to prevent repeated explosions.