American officials are taking a close look at Tsarnaev's possible contact with William Plotnikov, a fellow boxer who converted to Islam. Plotnikov joined the militant insurgency in the North Caucasus and was later killed in a raid by Russian authorities. Tsarnaev left the capital of Dagestan shortly after his death.
Authorities are also investigating what Tsarnaev was doing with a known militant recruiter in the region named Mansur Mukhamed Nidal, who was also killed in a raid. Officials say Tsarnaev was repeatedly seen leaving a controversial mosque in Makhachkala.
The twin blasts at the Boston Marathon bombing killed three people and wounded 170 others. Tamerlan and Dzhokhar Tsarnaev were named suspects based on surveillance footage. Tamerlan was killed in a shootout with police days later, while Dzhokhar was captured alive.
The FBI investigation into the April 15 attack is expanding, and almost a dozen individuals are now considered to be persons of interest, according to ABC News. Sources say female DNA was found on one of the bombs, but it was unclear whether it was from a victim, from someone who handled components of the bomb or from a possible co-conspirator.
On Monday, agents were seen leaving the home of Katherine Russell, the widow of Tamerlan Tsarnaev, carrying evidence bags. Sources say the FBI obtained samples of her DNA.
Russell's attorney insists she did not know about the bomb plot.
"In the coming days, Katherine will continue to meet with law enforcement, as she has done for many hours over the past week, and provide as much assistance to the investigation as she can," the attorney said in a statement.
Dzhokhar Tsarnaev is still recovering from his injuries and has stopped cooperating with authorities.
ABC News contributed to this report.