Russian officials say a male suicide bomber is responsible for Monday's explosion on a bus in the southern city of Volgograd that killed 14 people, one day after a female suicide bomber killed 17 people at the city's train station.
Authorities say about 20 people were wounded in Monday's powerful blast that occurred on a crowded trolleybus during the early morning rush hour. The attack destroyed the bus and blew out the windows of nearby buildings.
There was no immediate claim of responsibility for either explosion. The attacks 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.
The Kremlin said Monday Russian President Vladimir Putin has ordered the country's counterterrorism agency to step up security in Volgograd and elsewhere across the country.
Authorities say Sunday's blast, which wounded dozens of people, was set off by a female suicide bomber 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.