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.