A second suicide bomb has hit the Russian city of Volgograd in two days, with the latest strike on a trolleybus killing at least 14 people.
The incident follows an attack yesterday on the central station in the city which claimed 17 lives.
Russians officials said security had been tightened across the city to guard against further attacks.
Government leaders have expressed concerns militant groups from the volatile north Caucasus region could be increasing their use of violence in the build up to the 2014 Winter Olympic Games.
The Games will begin in the nearby city of Sochi in February.
Police have cordoned off the site of the second explosion as they looked for clues.
Both incidents have been described as acts of terrorism.
“It is now possible to preliminarily say that the explosive device was set off by a suicide bomber - a man whose body fragments have been collected and sent for genetic testing,” the Investigative Committee said in a statement to the media.
No group has yet said it was behind the blasts.