In this photo made by a public camera and made available by the Associated Press Television News the flash of an explosion illuminates the entrance to Volgograd railway station in Volgograd Russia on Sunday, December 29, 2013. More then a dozen people were killed and scores were wounded Sunday by a suicide bomber at a railway station in southern Russia, officials said, heightening concern about terrorism ahead of February's Olympics in the Black Sea resort of Sochi. (Photo by AP Photo/Associated Press Television News)

In this photo made by a public camera and made available by the Associated Press Television News smoke pours out after an explosion at Volgograd railway station, in Volograd Russia on Sunday, December 29, 2013. (Photo by AP Photo/ Associated Press Television News)

A police officer guards a main entrance to the Volgograd railway station hit by an explosion, in Volgograd, Russia, Sunday, December 29, 2013. (Photo by AP Photo)

The body of a victim lies at the site of a bomb explosion at the entrance to Volgograd railway station in Volgorgrad, Russia, 29 December 2013. At least 14 people were killed and 50 injured when a female suicide bomber blew herself up at the main railway station in the southern Russian city of Volgograd. The blast raised fears about terrorism attacks some six weeks before the opening of the Winter Olympic Games in the Black Sea resort of Sochi, some 700 kilometres south of Volgograd. (Photo by EPA/STR)

Russian security forces stand guard outside a train station following a suicide attack in the Volga River city of Volgograd, about 900 kms (560 miles) southeast of Moscow, on December 29, 2013. (Photo by Vladimir Pakhomov/AFP Photo)

Russian policemen stand guard next to a body at the explosion site on the entrance of the Volgograd railway station in Volgograd, Russia, 29 December 2013. (Photo by EPA/STR)

A bag lies on the ground as Russian security personnel inspect the damage at a train station following a suicide attack in the Volga River city of Volgograd, about 900 kms (560 miles) southeast of Moscow, on December 29, 2013. (Photo by AFP Photo/Stringer)

Russian firefighters and security personnel inspect the damage at a train station following a suicide attack in the Volga River city of Volgograd, about 900 kms (560 miles) southeast of Moscow, on December 29, 2013. (Photo by AFP Photo/Stringer)

Russian police investigators collect evidence following a suicide attack at a train station in the Volga River city of Volgograd, about 900 kms (560 miles) southeast of Moscow, on December 29, 2013. (Photo by AFP Photo/Stringer)

Ambulances line up a site of a trolleybus explosion, background, in Volgograd, Russia Monday, December 30, 2013. A bomb blast tore through the trolleybus in the city Volgograd on Monday morning, killing at least 10 people a day after a suicide bombing that killed at 17 at the city’s main railway station. (Photo by Denis Tyrin/AP Photo)

Dead bodies lie at the scene where a destroyed trolleybus stands on a street in Volgograd on December 30, 2013 after ten people were killed in a bombing on the packed vehicle, the second attack in the city in two days after a suicide strike on its main train station, officials said. The new attack will further heighten fears about security at the Winter Olympic Games which are due to open on February 7 in Russia's Black Sea resort of Sochi, which lies 690 kilometres (425 miles) southwest of Volgograd. (Photo by Kirill Braga/RIA Novosti)

A demolition expert clad in a special gear walks along a wreckage of a trolleybus in Volgograd, Russia Monday, December 30, 2013. (Photo by Denis Tyrin/AP Photo)

Military vehicles surround a wreckage of a trolleybus, in Volgograd, Russia, Monday, December 30, 2013. A bomb blast tore through a trolleybus in the city of Volgograd on Monday morning, killing at least 10 people a day after a suicide bombing that killed at least 17 at the city's main railway station. Volgograd is about 650 kilometers (400 miles) northeast of Sochi, where the Olympics are to be held. (Photo by Denis Tyrin/AP Photo)

An investigator takes photos in a wreckage of a trolleybus in Volgograd, Russia Monday, December 30, 2013. (Photo by Denis Tyrin/AP Photo)

Members of the emergency services work at the site of a blast on a trolleybus in Volgograd December 30, 2013. (Photo by Sergei Karpov/Reuters)

Investigators work at the site of a blast on a trolleybus in Volgograd December 30, 2013. (Photo by Reuters/Stringer)

Russian policemen guard the site of a deadly bombing on a packed trolleybus in Volgograd on December 30, 2013. (Photo by AFP Photo)

Members of the emergency services work at the site of a bomb blast on a trolleybus in Volgograd December 30, 2013. (Photo by Sergei Karpov/Reuters)