WASHINGTON -- Female DNA has been found on a fragment of one of the bombs that exploded at the Boston Marathon, sources tell CBS News.
While the DNA could have come from a marathon spectator or a clerk who sold the bomb-making materials, investigators say it's possible it came from a female accomplice.
Investigators will compare that against the DNA found on the bomb remnant to determine if she ever came in contact with the device.
Russell's family has issued a statement saying they were shocked by the attack. Still, investigators want to question Russell to find out if she had any knowledge of her husband's plans.
The FBI is also taking a fresh look at Tamerlan Tsarnaev's six-month trip to Russia and Dagestan in 2012 to determine if he received training from known terrorists. Of particular interest is a Russian-born Canadian militant named William Plotnikov.
Like Tsarnaev, Plotnikov was a boxer, and both men were in Dagestan last summer. Plotnikov was killed there in a shootout between Muslim radicals and police in July 2012, just days before Tsarnaev returned to the United States.
So far, investigators do not know if the two men ever met.
Surviving bombing suspect Dzhokhar Tsarnaev may have some answers, but has not talked to investigators since being read his rights a week ago. He remains in a 10-by-10-foot cell at the Federal Medical Center Devens, 40 miles outside of Boston.
Investigators have also found the mystery man named Misha, who the Tsarnaev family blamed for helping to radicalize Tamerlan.
The FBI has now interviewed Misha, and sources say there's no evidence he's connected to the Boston attack.
The defense team representing the Boston Marathon bombing suspect got a major boost Monday with the addition of Judy Clarke, a San Diego lawyer who has won life sentences instead of the death penalty for several high-profile clients, including the Unabomber and the gunman in the rampage that injured former Arizona Rep. Gabrielle Giffords.
Clarke's appointment was approved Monday by U.S. Magistrate Judge Marianne Bowler.
The judge denied, at least for now, a request from Miriam Conrad, the public defender of 19-year-old suspect Dzhokhar Tsarnaev, to appoint a second death penalty lawyer, David Bruck, a professor at Washington and Lee University School of Law.
Tsarnaev has been charged with using a weapon of mass destruction during the April 15 marathon. Three people were killed and more than 260 injured when two bombs exploded near the finish line.
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The suspect's lawyers could renew their motion to appoint another death penalty expert if he is indicted, the judge said.
Clarke's clients have included the Unabomber, Ted Kaczynski; Susan Smith, who drowned her two children; Atlanta Olympics bomber Eric Rudolph; and most recently Tucson, Ariz., shooter Jared Loughner. All received life sentences instead of the death penalty.
Clarke has rarely spoken publicly about her work and did not return a call seeking comment Monday. However, at a speech Friday at a legal conference in Los Angeles, she talked about how she had been "sucked into the black hole, the vortex" of death penalty cases 18 years ago when she represented Smith.
"I got a dose of understanding human behavior, and I learned what the death penalty does to us," she said. "I don't think it's a secret that I oppose the death penalty."
Harvard Law School professor Alan Dershowitz said the decision to put Clarke on Dzhokhar Tsarnaev's defense team shows "they are going to litigate hard against the death penalty."
"They are not going to put on a jihadi defense," he said. "The client wants to live, and he wants to avoid the death penalty. They are not going to say, 'I want to die, I want to join my brother.' "
Bruck has directed Washington and Lee's death penalty defense clinic, the Virginia Capital Case Clearinghouse, since 2004.