Russian officials and experts have claimed that rebels in Chechnya had close links with al-Qaida. They say dozens of fighters from Arab countries trickled into Chechnya during the fighting there, while some Chechen militants have fought in Afghanistan.
President Vladimir Putin's spokesman, Dmitry Peskov, said Friday that the Russian leader had long warned the West about the dangers posed by the Chechen rebels.
"Back at the time when we had a war raging in the Caucasus, Putin repeatedly said that the terrorists shouldn't be divided into 'ours' and 'theirs,' they mustn't be played with, differentiated into categories," Peskov said, according to Russian news agencies. It was an apparent reference to Western reluctance in the past to agree to the Kremlin branding rebels in Chechnya as terrorists.
The U.S. has long urged Russia's government and separatist elements in Chechnya not aligned with al-Qaida or other terrorist organizations to seek a political settlement.
Washington provided aid to the area during the high points of fighting in the 1990s and in the early 2000s, and has demanded human rights accountability. But the U.S. always backed the territorial integrity of Russia, never endorsing the separatists' desire for an independent state. And it has supported Russia's right to root out terrorism in the region.
Dozens of Chechens have trained in Pakistan's northwest frontier of Waziristan, but most have returned to Russia to fight.
Recently, however, the al-Qaida inspired group Tehrik-e-Taliban Pakistan has made strides at recruiting European fighters for attacks against the West, according to Noman Benotman, a former jihadist fighter who now works for the London-based Quillium Foundation.
The TTP, which has supported attacks in response to U.S. drone strikes, was linked to the failed 2010 attack in New York City's Times Square.
In recent years, people from Chechnya have faced charges in several European countries.
In 2011, a Danish court sentenced a Chechen-born man to 12 years in prison for preparing a letter bomb that exploded as he was assembling it in a Copenhagen hotel.
Last month, Spain's Interior Ministry said French and Spanish police arrested three suspected Islamic extremists in an operation in and around Paris. A statement said the suspected activists were of Chechen origin and believed linked to a terror cell dismantled in August in southern Spain.
That same month, a Turk and two Russians of Chechen descent were arrested and jailed in Spain on charges of belonging to an unidentified terror organization and possession of explosives. They have since been released while investigations continue.
The U.S. security think tank Stratfor said Friday that if the Tsarnaev brothers had any link to al-Qaida, or one of its franchise groups, it would "likely be ideological rather than operational, although it is possible that the two have attended some type of basic militant training abroad."
Stratfor added that the Boston bombings highlighted the fact that "the jihadist threat now predominantly stems from grassroots operatives who live in the West rather than teams of highly trained operatives sent to the United States from overseas, like the team that executed the 9/11 attacks."
"There will always be plenty of soft targets in a free society, and it is incredibly easy to kill people, even for untrained operatives," it said.
Isachenkov reported from Moscow. Also contributing to this report were Musa Sadulayev in Grozny, Russia; Bradley Klapper in Washington; Eric Tucker in Montgomery Village, Md.; Angela Charlton in Paris; Paisley Dodds in London; and Harold Heckle in Madrid.