(CNN) -- A new YouTube video has surfaced, allowing the public to hear the surviving Boston Marathon bombing suspect for the first time.
Dzhokhar Tsarnaev is seen playing with his niece in the video.
(To watch a portion of the YouTube clip, click on the video player above.)
Meanwhile, federal agents 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 was 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. 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, 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.
The source spoke the same day investigators moved forward on another front in Rhode Island, searching the family home of Tsarnaev's widow, Katherine Russell, for about 90 minutes.
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.
Agents left the home with items that included a black case and a clear plastic bag identified as DNA samples.
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, 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.
The second official warned that even if Russell's DNA matches the female DNA on the pressure cooker, that does not necessarily prove she had anything to do with the preparation of the bomb. She -- or any other female -- might have come into contact with the cooker in the past.
The DNA could also be from one of the victims, Lawrence Kobilinsky, a DNA expert at the John Jay College of Criminal Justice in New York, told CNN's Erin Burnett.
"It certainly is possible that it came from one of the victims," he said. "You have to interpret what we see."
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."
The two were married on June 21, 2010.
The elusive "Misha" interviewed
Also Monday, 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.
The double bombing near the finish line of the Boston Marathon on April 15 killed three people and wounded more than 260. Twenty-three remain hospitalized. At least 14 people have needed amputations, according to medical officials.
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 attack.
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, Tsarnaev'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.
Death-penalty expert to defend bomb suspect
Tsarnaev, 26, died after a shootout with police. Dzhokhar, 19, sustained gunshot wounds and is being held at a prison medical facility west of Boston. He has been charged with using and conspiring to use a weapon of mass destruction resulting in death, as well as one count of malicious destruction of property by means of an explosive device resulting in death.
On Monday, a federal judge appointed prominent defense lawyer Judy Clarke to represent the wounded suspect, who could be sentenced to death if convicted.
Legal colleagues consider Clarke to be the nation's foremost expert on defending federal capital cases. She has represented numerous high-profile clients facing Death Row, including Ted Kaczynski, the Unabomber; Eric Rudolph, who admitted to the 1996 Atlanta Olympic bombing and other attacks; and Jared Lee Loughner, who pleaded guilty to killing six people and wounding 13, including then-U.S. Rep. Gabrielle Giffords, in a Tucson, Arizona, shooting spree. All of them are serving life in prison.
Landfill search turns up empty
Investigators searched a landfill in New Bedford, Massachusetts, in hopes of finding a laptop that could be relevant to the case. But the two-day effort ended without success, a U.S. law enforcement official told CNN Monday.
The FBI was following leads from Dzhokhar and others that his laptop was thrown in a Dumpster and then picked up for disposal at a landfill.
The laptop might not be crucial to the investigation, the official added.
The suspects allegedly used low-grade explosives inside pressure cookers.
Investigators so far have found no evidence that the Tsarnaev brothers tested such bombs in the United States, the U.S. law enforcement official told CNN Monday.
If Tamerlan Tsarnaev received training in making bombs, it may have come during his trip to Russia, the official said.
Sources: Russian forces kill two jihadists
Investigators are looking into whether Tamerlan Tsarnaev was influenced toward radicalization during a six-month visit in 2012 to Dagestan, a region known to include radical jihadists who have battled the Russian government.
Russian special forces killed two members of a jihadist group in an early morning raid last weekend in the semiautonomous republic, two Russian police sources told CNN on Monday.
Authorities have not said whether the raid was linked to the Boston bombing.
But one of those killed was an associate of Abu Dujan, the slain leader of a militant Islamist organization that produced at least one video that Tamerlan Tsarnaev apparently posted and later removed from a social media account, according to an analysis by CNN and the SITE Intelligence Group.
'Jihad' discussed in wiretapped phone call
Russian authorities intercepted a phone call in early 2011 from one of the Tsarnaev brothers in the United States to their mother in Dagestan, an official with knowledge of the investigation told CNN over the weekend.
The wiretapped communication discussed jihad, but the conversation was vague, two U.S. officials said. It's unclear why the Russians were eavesdropping on the mother or for how long.
CNN has previously reported that the FBI conducted an investigation -- including an interview with Tamerlan Tsarnaev -- after Russia expressed concerns in 2011.
The Russians also raised questions about Tsarnaev's mother, Zubeidat Tsarnaev, according to several sources. Her name was subsequently added to a terrorism database along with her son's, an intelligence official said last week.
The FBI said at the time that it found nothing to justify further investigation and that Russia did not respond to U.S. requests for more information. The case was closed after several months.
One of the officials declined to say whether the information from the wiretapped phone call would have made a difference in uncovering plans for a future attack on the United States.
However, CNN contributor Tom Fuentes said the FBI would have found that information helpful after Russian officials asked the agency to look at Tamerlan Tsarnaev for signs of a possible shift toward increasing Islamic extremism.
Mother vows to travel to U.S. if she can see her son
On Monday, Zubeidat Tsarnaev told CNN's Nick Paton Walsh that she plans to come to the United States if she can see her son, despite pending shoplifting charges against her in Massachusetts.
Dzhokhar is at Federal Medical Center Devens. Authorities moved him there last week from Boston's Beth Israel Deaconess Medical Center.
He had what appeared to be gunshot wounds to his head, neck, legs and hand when he was captured on April 19 after a nearly 24-hour manhunt, according to the criminal complaint accusing him in the marathon blasts.
He is able to speak and has been interacting with staff, a prison spokesman said.
He has, however, apparently been less talkative since authorities read him his Miranda rights three days after his capture.
But the information gained from two sessions of questioning has produced good leads, a U.S. law enforcement official said.
CNN's Deborah Feyerick, Bill Mears, Susan Candiotti, Ashleigh Banfield, Paula Newton, Drew Griffin, Dave Alsup, Carol Cratty, Brian Todd and Barbara Starr contributed to this report.
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