An explosion ripped through a train station in the southern Russian city of Volgograd on Sunday, killing at least 10 people, Russian news agencies said, according to Reuters. The cause of the blast was not immediately clear. An attack by a female suicide bomber in the same Russian city on Oct. 21 killed seven people. On Friday, a car bomb killed three people in the southern Russian city of Pyatigorsk. Volgograd is a city of around 1 million people, about 430 miles (690 km) northeast of Sochi.