Very preliminary talks
While the elder Tsarnaev died April 19 after a firefight with police, his 19-year-old brother Dzhokhar is being held at a federal Bureau of Prisons medical center in Devens, Massachusetts, on a charge of using a weapon of mass destruction. He faces a possible death penalty if convicted.
On Monday, a federal judge appointed prominent defense lawyer Judy Clarke to represent him.
Clarke has represented Ted Kaczynski, the Unabomber; Eric Rudolph, the 1996 Atlanta Olympics bomber, and Jared Lee Loughner, who pleaded guilty in the Tucson, Arizona, shooting that killed six and left then-U.S. Rep. Gabrielle Giffords seriously wounded.
Federal prosecutors and the defense team of Dzhokhar Tsarnaev have had some very preliminary talks over the "past few days" to potentially allow the surviving suspect to resume providing information to investigators, possibly in exchange for eliminating the possibility of a death penalty if he is convicted, two government sources told CNN on Tuesday.
The communications are in the very early stages, and not a sign lawyers for either side is ready to make a deal, said one source, who did not want to be identified because of the sensitive nature of the private discussions. The source stressed these are not formal talks -- with a U.S. Justice Department official saying it would not be accurate to refer to the conversations as negotiations.
Prosecutors have not said they will definitely seek the death penalty in the case, but it is an option under the federal law the 19-year-old is accused of violating. Attorney General Eric Holder would have the final say.
These kinds of conversations are not unusual in such high-profile cases, legal sources say.
Russian cooperation, links
Over the weekend, news emerged that Russian authorities had intercepted a phone call in early 2011 from one of the Tsarnaev brothers in the United States to their mother in Dagestan. The call included a vague discussion of jihad, an official with knowledge of the investigation told CNN.
That information didn't make its way to the FBI before the bombings, the official said.
Last week, Russian President Vladimir Putin said he had regrets Russian intelligence wasn't able to provide more information to U.S. officials before the bombing, and said he hoped the incident would spark greater cooperation between U.S. and Russian counterterrorism officials.
Russia has been "very cooperative with us since the Boston bombing," Obama said Tuesday.
"Obviously, old habits die hard," he said. "There's still suspicions sometimes between our intelligence and law enforcement agencies that date back in some cases 10, 20, 30 years, to the Cold War."
Another possible investigative development tied to Russia relates to possible links between Tamerlan Tsarnaev and a Canadian jihadist killed by Russian troops in 2012, a source being briefed on the investigation said.
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.