At least seven people were killed on Monday after a suspected female suicide bomber detonated an explosive device on a bus in the southern city of Volgograd.

      Preliminary investigation revealed the attack was carried out by by Naida Asiyalova, a 30-year old woman from Russia’s republic of Dagestan, in the North Caucasus, which has been riven by unrest since the mid-1990s.

      The bomber attack took place on October 21, at about 2 p.m. local time (10:00 GMT) in the Kransoarmeisk district in Volgograd. According to witnesses, the explosion took place “almost immediately after” the woman got on the bus. Six people were found dead at the scene, as well as the body of the suicide bomber, another person has died in hospital.

      By evening, 28 people had been taken to hospital, including eight in serious condition, a city public health official told the press. Around 50 people were believed to be riding the bus at the time of the blast.

      The Investigative Committee said it has opened a case on multiple murder, terrorism-related and arms trafficking charges. No terrorist group has yet claimed responsibility for the attack.