Deadly suicide bombings in southern Russia have raised the spectre 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 bombings on Sunday and yesterday underscored the threat the country faces from a radical Islamic insurgency in the North Caucasus that has periodically spilled into the Russian 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 bombing at the Boston Marathon in April, but never before has an Olympic host country experienced terrorist violence on this scale in the run-up to 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 they were more concerned about security in Russia during the Sochi Games than they have been about any other Olympics since Athens in 2004.
The blast on Sunday occurred in the main railway station in Volgograd, a city 885 kilometres south of Moscow and 644 kilometres northeast of Sochi. That was followed by a blast that tore through an electric bus in the same city during rush hour yesterday morning, killing 14.
Both were carried out by suspected suicide bombers.
In October, a woman detonated a vest of explosives aboard a bus in the city, killing herself and six others.
It is not clear why suicide bombers have now chosen targets in Volgograd, a city of one million that was formerly called Stalingrad and is the site of one of the crucial battles of the second world war. It is the nearest major Russian city to the Caucasus, and its proximity may play a role.
The autonomous republics of the North Caucasus, including Dagestan and Chechnya, have been affected for nearly two decades by armed insurgencies.
The clashes are complex, ever-shifting conflicts that the International Crisis Group recently called "the most violent in Europe today".
The violence has claimed hundreds of lives this year, prompting the Russian authorities to make extraordinary efforts to keep it from reaching Sochi, a resort city on the Black Sea. The city will effectively be locked down from January 7.