The FBI statement on Friday that it interviewed Tamerlan Tsarnaev in 2011, at the request of a foreign government which it did not identify, was the first evidence that the family had come to security officials' attention after they emigrated to the US about a decade ago.
The brothers spent their early years in a small community of Chechens in the central Asian country of Kyrgyzstan, a mainly Muslim nation of 5.5 million. The family moved in 2001 to Dagestan, a southern Russian province that lies at the heart of a violent Islamist insurgency and where their parents now live.
In separate interviews, the parents of the Tsarnaev brothers said they believed their sons were incapable of carrying out the bombings. Others remembered the brothers as friendly and respectful youths who never stood out or caused alarm.
"Somebody clearly framed them. I don't know who exactly framed them, but they did. They framed them. And they were so cowardly that they shot the boy dead," father Anzor Tsarnaev said in an interview in Dagestan's provincial capital, Makhachkala, clasping his head in despair.
The mother, Zubeidat Tsarnaev, told Russia Today state television: "It's impossible, impossible, for both of them to do such things, so I am really, really, telling that this is a setup."
Tamerlan Tsarnaev had studied accounting as a part-time student at Bunker Hill Community College in Boston for three semesters from 2006 to 2008, the school said. He was married with a young daughter.
Dzhokhar Tsarnaev was registered as a student at the University of Massachusetts Dartmouth. Students said he was on campus this week after the Boston Marathon bombing. The campus closed down on Friday along with colleges around the Boston area, and it remained closed yesterday as law enforcement continued investigating.
The men's father, Anzor Tsarnaev, said in a telephone interview from the Russian city of Makhachkala that his younger son, Dzhokhar, is "a true angel". He said his son was studying medicine.
"He is such an intelligent boy," the father said. "We expected him to come on holidays here."
The city of Cambridge announced two years ago that it had awarded a $2,500 scholarship to Dzhokar Tsarnaev. He was then a senior at Cambridge Rindge & Latin School, a highly regarded public school whose alumni include Matt Damon and Ben Affleck.
Ruslan Tsarni, the young men's uncle, called his nephews "losers" and said they had struggled to settle in the US and ended up "thereby just hating everyone".