MOSCOW (AP) - Russian investigators say a female suicide bomber has struck a passenger bus in the southern city of Volgograd, killing five people and injuring at least 20 others.

The state ITAR-Tass news agency cited Valery Safonov, an official with Russia's main investigative agency, saying the suspected bomber was the partner of a militant. He did not say where that militant was from.

The spokesman for the Investigative Committee confirmed that the explosion was an act of terrorism.

Monday's explosion was the first on a bus in Russia since a 2008 bombing carried out by a female suicide bomber in the North Caucasus, a region in southern Russia where an Islamic insurgency is simmering.

A total of 40 people were on the city bus at the time of the explosion, said Irina Gogolyeva, a spokeswoman for the Emergency Situations Ministry. Many of the injured were hospitalized in serious condition, ITAR-Tass reported.

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