(CNN) -- At least five people were killed and more than 20 were injured Monday afternoon by what appears to be a suicide bombing on a passenger bus in the southern Russian city of Volgograd, authorities said.
Early indications are that a female suicide bomber set off the blast, Valery Safonov, a spokesman for the Volgograd region's top investigative agency, told state-run TV outlet Russia 24.
The vehicle was at the Lesobaza bus stop in Volgograd when the explosion happened at about 2 p.m., the Investigative Committee of the Russian Federation said.
Seven of the injured were in serious condition, the committee said.
A criminal investigation has been launched, and bomb technicians were at the blast site Monday afternoon, according to the committee.
The committee said five people were killed. Earlier, the Russian interior ministry's press service said six were killed.