Whatever Tamerlan Tsarnaev’s affiliations may have been, they are still unknown and being investigated. While the mainstream media has trumpeted that the Tsarnaev brothers were Chechens, they were in fact originally from Dagestan, a region immediately to the north and east of Chechnya, and their family had since moved to Kyrgyzstan. Of course, separating Dagestan and Chechnya in some ways is as artificial as separating Afghanistan and Pakistan; there is considerable ethnic spillover, and Chechen Islamist activity in Dagestan goes back quite a ways.
The regions of Dagestan bordering Chechnya have a large population of Akkin Chechens, and as early as 1997, the Chechen jihadists were trying to agitate the Dagestani Chechens into joining their jihad against Russia. For the most part, these Akkin Chechens did indeed support the Caucasus-wide jihad. It should be noted that most of the Muslims in the Caucasus region are Sufis (who are considered heretics by most Sunni and Shia Muslims), and that they lean toward more of a philosophy based on pacifism and mysticism. However, Soviet anti-religious activity had the effect of rendering a lot of the sheikhs religious, while actually being ignorant of their own theology. This opened the door to the jihadists in later years.
Many of the al-Qaeda affiliated Islamist groups in Chechnya and Dagestan are grouped together in the Caucasus Emirate, led by Doku Umarov, who masterminded the Beslan massacre. One of these groups, Vilayet Dagestan, issued a statement about the Boston bombing on April 21st. In their statement, they neither confirmed nor denied that Tsarnaev (who did travel to Dagestan last year) had any affiliation with them, but simply stated that they are at war with Russia, not the United States. They went on to suggest that the U.S. should look into the Russian Security Forces, implying that the Russians were responsible for the attack. They also said that Umarov had ordered them not to attack civilians. (The same Umarov who was responsible for the massacre of schoolchildren in Beslan.)
Terrorist groups are not known for their veracity. The pattern of blaming their enemy for their own actions has been seen before; some Al Qaeda-affiliated groups have blamed 9/11 on the US government, for example. (They’re not the only ones, but the tinfoil hat brigade doesn’t enter into this article.) Some of this is pure propaganda. Many Islamist terror groups present themselves as victimized resistance, and taking offensive action against civilians would damage that image. Furthermore, Tsarnaev’s YouTube channel included several videos from Vilayet Dagestan, suggesting that there may have been a link that they at present do not want to acknowledge.