The autonomous republics of the North Caucasus, including Dagestan and Chechnya, have been roiled for nearly two decades by armed insurgencies — 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 coast among the foothills of the Caucasus. The city will effectively be locked down starting Jan. 7, with all traffic banned except officially registered vehicles. Visitors to the Olympics, which begin on Feb. 7, will be required to obtain a special pass to enter the region.

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. On Friday, an explosion in a car killed three people in Pyatigorsk, another city in the Caucasus; details of that attack remain sketchy, and it was not clear whether it was related to Sunday’s bombing.

Doku Umarov, the Chechen rebel fighter who now leads a terrorist group known as the Caucasus Emirate, vowed in July to attack in Sochi, calling the Games “satanic.”

“They plan to hold the Olympics on the bones of our ancestors, on the bones of many, many dead Muslims, buried on the territory of our land on the Black Sea,” Mr. Umarov said in a video. He emerged from the ruins of Chechnya’s separatist movement, which was largely defeated by the authorities under Mr. Putin; he has claimed responsibility for two suicide bombings in Moscow in 2010 and 2011.

One reason American 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 security resources to the Greek government in 2004, but the Russians have stronger capabilities and almost always refuse American offers of help.

Current and former American officials said they had confidence in the Russians’ ability to protect Sochi and the Olympic venues. “There’s every belief they’ll make it secure and do whatever it takes to do that,” said one senior law enforcement official. “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.”

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 American officials see terrorists in countries like Pakistan and Yemen posing a greater threat to American 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.

“We share some enemies, but not that many,” Mr. Nelson said. “The U.S. just doesn’t put the resources into those areas like it does elsewhere in the world, and there isn’t a ton of intelligence sharing on these groups.”