Suicide Bomber Strikes Trolley Bus in Russia

Russian President Vladimir Putin ordered security to be tightened around the country following the second attack, which comes less than six weeks before the games are set to start in the Black Sea resort of Sochi, raising questions whether the violence could threaten to tarnish an event on which Mr. Putin has staked his personal pride and spent $50 billion to stage.

The games have already weathered heavy criticism abroad due to the recent passage of a controversial law banning gay "propaganda" which has led to calls for a boycott among gay rights groups.

A top rebel leader in the county's restive North Caucasus region has called for attacks in the run-up to the event, and the repeated targeting of Volgograd—which is situated about 680 kilometers (425 miles) from the Olympic site—appears aimed at raising anxiety before the games, which Russia has poured significant resources into protecting.

The latest attacks were the deadliest in the country in nearly three years. Though there was no immediate claim of responsibility, authorities said initial indications suggested the blasts were the work of Islamist terrorists from the volatile North Caucasus region.

"This is about embarrassing the Russian government and creating a state of insecurity," said Matthew Clements, a Russian defense and security expert at IHS Country Risk. "The chances of a successful attack at one of the Olympic venues are slim, but attacks in the region around it still have the desired psychological effect."

Normally stoic Russians reacted uneasily in the wake of the second attack. Russian Railways announced it was imposing "total inspections" at stations around the country during the busiest travel days of the year ahead of the long New Year's holiday. In St. Petersburg, city officials canceled a New Year's Eve fireworks display due to security concerns and authorities briefly closed Red Square and at least one Moscow metro station following reports of suspicious packages.

Russia is deploying tens of thousands of police and security forces to protect the games and the country's Olympic Committee Chairman Alexander Zhukov told Russian news agencies that there was no need to increase security measures surrounding the games as "all necessary steps have already been taken."

No other Olympic Games has ever been held so close to an area of conflict and the attacks raised questions yet again about whether Sochi was a problematic choice for such a high-profile international event. On Monday, Russian anti-terrorist forces said they killed three militants in the Kabardino-Balkaria Republic just 320 kilometers east of Sochi, who they suspected of plotting a New Year's attack. On Friday, a car bomb outside a police station killed three people in the southern Russian city of Pyatigorsk, which is about 275 kilometers from Sochi.

International Olympic Committee spokesman Mark Adams said in an email that the organization doesn't comment "on the detail of security issues" and that the matter "is very much something that we leave in the hands of the authorities."

"It was a risky proposition to put the games so close to an area where there is an established network of insurgents," Mr. Clements said. "The threat of terrorism has existed with other games, but the fact that Sochi is taking place very close to an area of long-running conflict sets it apart."

Vladimir Markin, a spokesman for Russia's Investigative Committee, said Monday morning's attack appeared to be the work of a male suicide bomber who was believed to have carried an explosive device packed with shrapnel and the equivalent of about 4 kilograms (9 pounds) of TNT. The rush-hour blast blew the roof off the blue-and-white trolley bus and shattered the windows of a five-story building opposite the scene.

There were conflicting reports on the death toll, with the Investigative Committee and the Health Ministry saying 14 were killed and the Emergency Situations Ministry saying 15 had died. Twenty-eight people were wounded by the blast, the Investigative Committee said.

Mr. Markin said the device used was of similar design to the one detonated by a suicide bomber in the portico of Volgograd's main railway station on Sunday, killing 17 people and wounding 45, leading investigators to believe the attacks were organized by the same people.

The twin attacks were the deadliest Russia has seen since a suicide attack in the arrival hall of Moscow's Domodedovo International Airport killed 37 people in January 2011. Ten months earlier, twin suicide bombings aboard packed trains in Moscow's metro system killed 40 people.

While Russia's security forces have struggled to suppress militant groups in the Caucasus region, Islamic terrorism has faded in the Russian heartland in recent years. Earlier this year, however, rebel leader Doku Umarov called for attacks against civilian targets in the run-up to the Sochi games. Volgograd was previously targeted in late October, when a female suicide bomber from the Dagestan region of the Caucasus killed seven people on a bus near a university.

"A terrorist attack there is an attack in the heart of Russia. To attack there is much more resonant than in the Caucasus and it is easier to hit than Moscow," said Vadim Mukhanov, a senior researcher at the Center on the Caucasus and Regional Security at the Moscow State Institute of International Relations.

Volgograd—a city of about one million—is Russia's closest major metropolis to the North Caucasus. It is also a key transit center connecting the south of the country—including the Caucasus region—with the rest of Russia. After Monday's attack, city authorities mobilized cadets from a local police academy and announced plans to bring in Cossack patrols to help maintain security.

Volgograd—previously known as Stalingrad—also is symbolic because of its importance to Russia's past as the site of a historic World War II battle in which Nazi Germany's advance into Russia was turned back.

"Volgograd, a symbol of Russia's suffering and victory in World War II, has been singled out by the terrorist leaders precisely because of its status in people's minds. Their aim is to hurt as many ordinary people as they can, and terrorize the rest," said Dmitri Trenin, director of the Carnegie Moscow Center.