(CNN) -- 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.