At least 14 people have been killed in a suicide bombing on a trolleybus in the Russian city of Volgograd, investigators say.
The blast comes a day after 17 people died in another suicide attack at the central station in the city.
Security has been tightened at railway stations and airports across Russia.
Moscow is concerned militant groups could be ramping up violence in the run-up to the 2014 winter Olympic Games in the city of Sochi.
The Olympic venue is close to Russia's volatile north Caucasus region.