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Experts and police officers examine a site of a trolleybus explosion in Volgograd, Russia Monday, Dec. 30, 2013. A bomb blast tore through the trolleybus in the city of Volgograd on Monday morning, killing at least 14 people a day after a suicide bombing that killed at 17 at the city's main railway station. (AP Photo/Denis Tyrin) ( Russia Explosion )

Victims of a terrorist attack lay at a site of an explosion at a trolleybus after an attack by a suicide bomber in Volgograd, Russia, on Dec. 30, 2013. Reports state that at least 17 people were killed and 28 wounded in the bombing, which investigators said bore a similar signature to attacks a day earlier and in late October 2013 in the city. EPA/STR ( RUSSIA VOLGOGRAD EXPLOSION )

Rescuers and ambulance doctors transfer injured victims of a bomb explosion at the train station of Volgograd from local hospitals to a medical aircraft to transport them to specialized hospitals in Moscow, at the Volgorad airport, Russia, early December 30. At least 17 people were killed by a suicide bomber who blew herself up at the main railway station in the southern city of Volgograd. EPA/EMERGENCIES MINISTRY VOLGOGRAD ( RUSSIA TERRORIST ATTACK )

A demolition expert clad in a special gear walks along a wreckage of a trolleybus in Volgograd, Russia Monday, Dec. 30, 2013. A bomb blast tore through the trolleybus in the city of Volgograd on Monday morning, killing at least 14 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. (AP Photo/Denis Tyrin) ( Russia Explosion )

Women cry while laying flowers outside the Volgograd main railway station in Volgograd, Russia, early Monday Dec. 30, 2013. Russian authorities ordered police to beef up security at train stations and other facilities across the country after a suicide bomber killed 14 people on a bus Monday in the southern city of Volgograd. It was the second deadly attack in two days on the city that lies just 400 miles (650 kilometers) from the site of the 2014 Winter Olympics. (AP Photo/Denis Tyrin) ( Russia Explosion )

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. At least 18 people were killed and dozens injured when a suicide bomber blew herself up in the train station ahead of February's Olympic Games in nearby Sochi. AFP/Getty Images ( 525678105 )

View of the wreckage of a trolleybus following an attack killing at least 14 people in the southern Russian city of Volgograd on December 30, 2013. The force of the blast left almost no trace of the trolleybus, which was packed with early morning commuters, and also blew out windows of nearby houses, state television said. The new attack will further heighten fears about security at the Winter Olympic Games which are set to open on February 7 in Russia's Black Sea resort of Sochi, southwest of Volgograd. AFP/Getty Images ( 525687403 )

People grieve at the site of the second terrorist explosion happened on a trolleybus, at a street in Volgograd, Russia. According to media reports, at least 31 people were killed and many were injured in two suicide bombing attack in the city during the last two days. EPA/MAXIM SHIPENKOV ( RUSSIA EXPLOSION AFTERMATH )

Russian police investigators collect evidence following a suicide attack at a train station in the Volga River city of Volgograd on Sunday, December 29, 2013. AFP/Getty Images ( 525679322 )

Russian people grieve at the site of the second terrorist explosion happened on a trolleybus, at a street in Volgograd, Russia, late Monday, Dec. 30, 2013. According to media reports, at least 31 people were killed and many were injured in two suicide bombing attack in the city during the last two days. EPA/MAXIM SHIPENKOV ( RUSSIA EXPLOSION AFTERMATH )

Russian workers (background) restore an entrance of the main railway station, the site of the first terrorist explosion in Volgograd, Russia. EPA/MAXIM SHIPENKOV ( RUSSIA EXPLOSION AFTERMATH )

Women grieve at the site of the second terrorist explosion happened on a trollybus, at a street in Volgograd, Russia, late Monday, Dec. 30. According to media reports, at least 31 people were killed and many were injured in two suicide bombing attack in the city during the last two days. EPA/MAXIM SHIPENKOV ( RUSSIA EXPLOSION AFTERMATH )

Women enter the Volgograd main railway station in Volgograd, Russia early Monday, Dec. 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. (AP Photo/Denis Tyrin) ( Russia Explosion )

The covered bodies of victims lie 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 on December 29, 2013. AFP/Getty Images ( 525678977 )

A Russian man mourns at the site of the second terrorist explosion happened on a trolleybus, at a street in Volgograd, Russia on Monday, Dec. 30. According to media reports, at least 31 people were killed and many were injured in two suicide bombing attack in the city during the last two days. EPA/MAXIM SHIPENKOV ( RUSSIA EXPLOSION AFTERMATH )

Remains of a trollybus litter the street following a suicide attack that destroyed the packed bus, killing 14 people in the southern Russian city of Volgograd. The deadly bombing was caused by a male suicide bomber, investigators said on December 30. The attack comes a day after 17 people died in a suicide strike on the city's main train station, exposing the huge challenge Russia's President Vladimir Putin faces in ensuring safety at Winter Olympic Games staged on the very edge of the violence-plagued North Caucasus. AFP/Getty Images ( 525688456 )

A Volgograd resident walks crying in Volgograd, Russia early Monday, Dec. 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 400 miles northeast of Sochi, where the Olympics are to be held. (AP Photo/Denis Tyrin) ( Russia Explosion )

A soldier guards the Volgograd main railway station in Volgograd, Russia early Monday, Dec. 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. (AP Photo/Denis Tyrin) ( Russia Explosion )

A police officer walks along a wreckage of a trolleybus in Volgograd, Russia, Monday, Dec. 30, 2013. A bomb blast tore through the trolleybus in the city of Volgograd on Monday morning, killing at least 14 people a day after a suicide bombing that killed at least 17 at the city's main railway station. (AP Photo/Denis Tyrin) ( Russia Explosion )