MOSCOW — A deadly suicide bombing at a crowded railroad station in southern Russia on Sunday, followed by a blast in a trolley bus on Monday in the same city, raised the specter of a new wave of terrorism just six weeks before the Winter Olympics in Sochi.
President Vladimir Putin’s government has worked to protect the Olympics with some of the most extensive security measures ever imposed for the Games. But the Volgograd bombings underscored the threat Russia faces from a radical Islamist insurgency in the North Caucasus that has periodically spilled into the heartland, with deadly results, including several recent attacks.
Security has become a paramount concern at all major international sporting events, especially in the wake of the bombings at the Boston Marathon in April, but never before has an Olympic host country experienced terrorist violence on this scale soon before the games. And would-be attackers may have more targets in mind than the Russian state.
Current and former US law enforcement and intelligence officials said Sunday that they were more concerned about security in Russia during the Sochi Games than they have been about any Olympics since Athens in 2004.
Russian officials attributed the explosion Sunday to a bomb packed with shrapnel, possibly carried in a bag or backpack. It was detonated in the main railroad station in Volgograd, a city 550 miles south of Moscow and 400 miles northeast of Sochi. The bomb blew out windows and left a horrific scene of carnage at its main entrance. At least 16 people were killed, and nearly three dozen others were wounded, some of them critically, meaning the death toll could still rise.
On Monday morning a second blast struck a trolley bus in the city, killing at least 10 people, according to preliminary reports. Photographs posted by Russian media showed that the force of the blast tore open the bus and shattered windows nearby. At least 10 others were wounded in what officials immediately described as another suicide bombing.
The Sunday blast, captured on a surveillance video camera from across the central plaza in front of the station, occurred near the station’s metal detectors, which have become common at most Russian transportation hubs. That raised the possibility that an attack deeper inside the station or aboard a train had been averted.
Vladimir I. Markin, a spokesman for the main national criminal investigation agency in Russia, called the bombing an act of terror, though the exact motivation, target, and perpetrator were not immediately clear.
Within hours of the attack, the authorities blamed a suicide bomber, and cited the gruesome discovery of the severed head of a woman, which they said could aid in identifying her as the suspect. Officials later said they had found a grenade and a pistol, and suggested that the attack might have been carried out by a man and a woman working together.
The attack was the second suicide bombing in Volgograd in recent months. In October, a woman identified as Naida Asiyalova detonated a vest of explosives aboard a bus in the city, killing herself and six others.
In that case, the authorities said she was linked by marriage to an explosives expert working with an Islamist group in Dagestan, a republic in southern Russia where the police have struggled to suppress a Muslim separatist insurgency. A month later, the authorities announced that they had killed her husband and four others in a raid. But the attack Sunday indicated that the threat was far from extinguished.
It was not clear why suicide bombers have chosen targets in Volgograd, a city of 1 million that was formerly called Stalingrad, the site of one of the crucial battles of World War II. It is the nearest major Russian city to the Caucasus, and its proximity may play a role.
Putin vowed Sunday to redouble security at Russia’s railway stations and airports, which are especially busy around the New Year’s holiday.
With security so tight at the site of the Games, experts have warned that insurgents who want to disrupt the Olympics might turn instead to “softer” targets elsewhere.
One reason US law enforcement and intelligence officials are concerned about Sochi is that the United States has more of an arms-length relationship with Russia than with most Olympic host countries. The United States provided extensive resources to the Greek government in 2004, but the Russians have stronger capabilities and almost always refuse American offers of help.
Current and former US officials said they had confidence in the Russians’ ability to protect Sochi and Olympic venues.
“But it is a large country, and these groups can get a lot of bang for their buck if they are able to do something in the country, wherever it is, during the Olympics,” said one senior law enforcement official.
Another complicating factor is that the United States does not direct a significant portion of its intelligence capabilities toward groups that mount attacks in Russia, because US officials see terrorists in countries like Pakistan and Yemen posing a greater threat to US interests. And even though the Boston Marathon bombers had ties to Dagestan, officials don’t believe the attack was plotted with the help of groups there.