Russian officials say a blast on a bus in the southern city of Volgograd has killed at least 10 people, just one day after a suicide bombing killed 17 people at the city's train station.
Authorities say at least 10 people were wounded in Monday's explosion, which officials say may have been caused by a bomb planted on the trolleybus.
The attack took place during the early morning rush hour, leaving the bus destroyed and blowing out windows of nearby buildings.
There was no immediate claim of responsibility for either explosion, which came just weeks before the Winter Olympics open in Sochi, about 650 kilometers southwest of Volgograd. Islamist militants have threatened to disrupt the Winter Games.
An investigative committee spokesman says a woman set off Sunday's bomb in front of metal detectors at the train station's entrance. Dozens of people were wounded.
Authorities found what they say was the bomber's severed head. They identify her as coming from Dagestan, a republic in the nearby volatile North Caucasus.
An attack in Volgograd by a female suicide bomber on October 21 killed five people and wounded 30. Investigators also identified her as coming from Dagestan.
Dagestan is the epicenter of an ongoing Islamist insurgency in the North Caucasus.
In early July, the leader of the Islamist insurgency in the North Caucasus, Doku Umarov, declared an end to a moratorium on attacks on Russian civilian targets that he had announced the previous year.