Investigators are looking into possible links between Tamerlan Tsarnaev, 26, and a Canadian boxer-turned-jihadist killed in a firefight with Russian forces in the southwestern republic of Dagestan in July 2012, while Tsarnaev was visiting the region, CNN reported citing sources.
Tamerlan Tsarnaev flew out of Dagestan two days after the burial of William Plotnikov, 23, who had been born in Russia, but his family moved to Canada when he was a teenager, the channel said. Investigators are looking into the possibility he left because of Plotnikov's death, CNN said citing sources.
The investigators are also looking into whether Tsarnaev had any contact with another militant named Mahmoud Mansur Nidal, 18, who was killed by Russian forces in May 2012 during a gun battle in Makhachkala, Dagestan's capital, according to CNN sources.
Tsarnaev's parents live in Makhachkala. Possible links between Tsarnaev and Plotnikov and Nidal were first reported by a Russian magazine, Novaya Gazeta, it said.
Meanwhile, FBI agents have interviewed the man identified as "Misha," an elusive figure whose name has surfaced in the Boston bombing investigation, CNN reported citing US officials.
Investigators spoke with the man in Rhode Island after reports surfaced suggesting that members of the suspected bombers' family blame a "Misha" for radicalising Tsarnaev, whose wounded brother has identified him as the mastermind of the April 15 bombing.
The man, whose real name is Mikhail Allakhverdov, denies ever encouraging a violent take on Islam and says he was not Tamerlan's teacher, according to a New York Review of Books writer, Christian Caryl who says he interviewed Misha.
Investigators also searched the family home of Tamerlan Tsarnaev's widow, Katherine Russell, in Rhode Island, for about 90 minutes on Monday.
Russell and her toddler daughter-Tamerlan's child-have been staying at the North Kingstown home with her parents. Her attorneys were present during the search.
Female DNA was discovered on a fragment of the pressure cooker bombs used in the attack and investigators are trying to determine whose genetic material it was, CNN reported citing law enforcement sources.
However, sources cited by the channel stressed the DNA could be from anyone who came in contact with the products used to make the bomb and it does not necessarily implicate anyone.
Russell has said she was completely in the dark about her husband's alleged plan. Her attorney said the news "came as an absolute shock."
A federal judge Monday appointed prominent defence lawyer Judy Clarke, who has represented numerous high-profile clients facing Death Row, to represent the surviving suspect, Tamerlan's younger brother Dzhokhar, 19, who could be sentenced to death if convicted.