It also appears that Tsarnaev's wife, Katherine Russell, began to dress in a conservative Muslim fashion, according to people in her family's neighborhood in Rhode Island.
But the picture of Tsarnaev's evolving views is far from complete. People who knew him have described him as an outsider, someone who never really felt at home in the United States. At one point, he said he had no American friends; he couldn't understand them.
The details that have been reported so far on the explosive devices used provide no firm indication of training.
According to investigators, the devices were rudimentary: low-grade explosives made more powerful by being inside pressure cookers, with nails attached to maximize their lethality. The other explosive devices allegedly in their possession -- pipe bombs and makeshift grenades -- also appear to have been basic in design.
According to explosive experts, such devices could be built by downloading instructions off the Internet and would not require training.
However, bomb-making manuals produced by al Qaeda and its sympathizers tend to contain many errors, according to Sidney Alford, a British explosives expert who has reviewed many such manuals.
Alford told CNN in 2011 that while the overall quality of the documents was low "some of the content is certainly technically viable and they do contain among some pages of rubbish quite practical means of causing havoc."
He said that a bomb-making novice with little chemistry background could be fortunate enough to select a recipe that works and succeed in making a viable device. "Once you've got the substances, you need only the most rudimentary equipment and no special facilities, no special place to do it," Alford said.
Al Qaeda in the Arabian Peninsula's English online magazine Inspire has included step-by-step bomb-making recipes. According to authorities, one such formula, "How to Make a Bomb in Your Mom's Kitchen," published in the summer 2010 edition of Inspire has been downloaded by Islamist militants plotting terrorist attacks in multiple plots on both sides of the Atlantic.
It is not yet clear whether the devices were detonated by timer or remote control. Instructions for how to build both are widely available on the Internet, though analysts say building a remote detonation device is significantly more challenging and may point to some overseas training.
An issue of Inspire magazine released in May 2012 provided a step-by-step guide on how to build remote control detonators to circumvent the need for formal training. The so-called Toronto 18, arrested in June 2006 on charges of plotting to blow up targets in Canada, successfully built a remote control mechanism to detonate their bombs without receiving training overseas.
For counterterrorism agencies, self-radicalization is difficult to trace because it may not involve obvious contacts with others.
Christopher Swift, professor of national security studies at Georgetown University, said that "if we are looking at a situation where there was a foreign mentor, or where one or more of the perpetrators may have gone to Russia to get some kind of training or indoctrination, then we're looking at something that has a much more international nexus than the sort of self-radicalization that we see within the United States within immigrant communities."
Swift added: "That's where global and local would come together. But we don't have enough information yet on that."
Whether Tsarnaev's return to Russia in the first half of 2012 changed his outlook is likely to be one of many avenues of inquiry. Before he left, the FBI had already -- at the request of an unnamed foreign government -- investigated possible links with militant groups but discovered nothing suspicious.
In a statement issued late Friday, the FBI said that "in early 2011, a foreign government asked the FBI for information about Tamerlan Tsarnaev ... based on information that he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country's region to join unspecified underground groups."
The FBI said it looked for such things as derogatory telephone communications, possible use of online sites associated with the promotion of radical activity, associations with other persons of interest, travel history and plans, and education history but "did not find any terrorism activity, domestic or foreign."
In the view of Swift, who studies Islamic radicalization, "This is two young men trying to find an identity. These are not people who fought in the war in Chechnya, they're not people who had any close, proximate relationship to that conflict."
Swift said, "There is no evidence that these young men were seeking to make a point about Chechnya per se."
He said he believes it is more likely that they were trying to make a point about themselves, as if to say: 'We are warriors. we have been wronged, people do not understand us and we must be heard.' "