BOSTON (CNN) -

Female DNA was discovered on a fragment of the pressure cooker bombs used in the Boston Marathon attack and investigators are trying to determine whose genetic material it was, law enforcement sources told CNN.

But one of the sources 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.

Federal agents also are looking into possible links between dead Boston Marathon bomb suspect Tamerlan Tsarnaev and a Canadian boxer-turned-jihadist killed by Russian troops in 2012, a source being briefed on the investigation said Monday.

William Plotnikov and six others died in a firefight with Russian forces in the southwestern republic of Dagestan in July 2012, while Tsarnaev was visiting the region, the source said. The 23-year-old Plotnikov had been born in Russia, but his family moved to Canada when he was a teenager.

The source said Plotnikov's body was prepared for burial by a local imam on July 14. Tamerlan Tsarnaev flew out of Dagestan two days later, arriving in New York on July 17. Investigators are looking into the possibility he left because of Plotnikov's death, the source said.

Additionally, the source says investigators are 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.

Tsarnaev's parents live in Makhachkala. Possible links between Tsarnaev and Plotnikov and Nidal were first reported by a Russian magazine, Novaya Gazeta.

And the source said that about a month before he returned to the United States, Tamerlan Tsarnaev applied for a Russian passport at a government office in Dagestan, telling authorities he had lost his existing passport. According to the source, Tsarnaev left Dagestan before his new passport arrived. It's not clear whether he traveled on an existing Russian or Kyrgyz passport.

That report emerged the same day a U.S. government official told CNN that FBI agents have interviewed the man identified as "Misha," an elusive figure whose name has surfaced in the Boston bombing investigation.

Investigators spoke with the man in Rhode Island after reports surfaced suggesting that members of the suspected bombers' family blame a "Misha" for radicalizing 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 who says he interviewed Misha.

"He began telling me he cooperated with the FBI" and had handed over his computer and cell phone, reporter Christian Caryl told CNN on Monday.

Allakhverdov insisted he had "nothing to do with radicalization," Caryl said.

CNN has made repeated efforts to speak with Allakhverdov, but has so far been unsuccessful.

A lawyer who stepped out of the West Warwick, Rhode Island, apartment listed for Mikhail Allakhverdov told CNN he represents the parents of someone who lives there, adding, "We call him Michael."

The parents have answered all questions the authorities have asked of them, attorney Richard Nicholson said.

The parents are nervous because of the media focus on them, he said, adding that the mother has a heart condition.

Misha's family 'friendly and welcoming'

Caryl said that when he showed up at Misha's home, he took the family by surprise but managed to spend some time with him.

"I wasn't his teacher. If I had been his teacher, I would have made sure he never did anything like this," Allakhverdov said, according to Caryl's report.

"A thirty-nine-year-old man of Armenian-Ukrainian descent, Allakhverdov is of medium height and has a thin, reddish-blond beard," Caryl wrote. "When I arrived he was wearing a green and white short-sleeve football jersey and pajama pants. Along with his parents, his American girlfriend was there, and we sat together in a tiny living room that abuts the family kitchen."

He added, "In many ways, Allakhverdov's parents seem typical former-Soviet émigrés who had embraced middle class life in the United States. His father is an Armenian Christian and his mother is an ethnic Ukrainian."

In the article, Allakhverdov's father is quoted as saying, "We love this country. We never expected anything like this to happen to us."

Ruslan Tsarni, Tamerlan's uncle, told CNN last week that a friend of his nephew "just took his brain. He just brainwashed him completely."

After The Associated Press said members of Tsarnaev's family identified the friend as Misha, Tsarnaev's former brother-in-law told CNN that Tsarnaev had a friend by that name. Elmirza Khozhgov said the friend apparently "had influence on Tamerlan." But Khozhgov said he did not see Misha try to radicalize Tsarnaev.