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.
Tsarnaev flew out of Dagestan two days after Plotnikov's body was prepared for burial, according to the source. Investigators are looking into the possibility he left because of Plotnikov's death, the source said.
Like Tamerlan Tsarnaev, Plotnikov was once a boxer.
Investigators also are looking into whether Tsarnaev had any contact with another militant named Mahmoud Mansur Nidal, who was killed by Russian forces in May 2012 during a gun battle in Dagestan's capital, the source said.