(CBS News) 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.
Katherine Russell, the widow of accused bomber Tamerlan Tsarnaev, is not a suspect in the bombings and has not been charged in the plot. However, FBI agents met with Russell at her parents' Rhode Island home Monday to collect a DNA sample.
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.
Watch: Why didn't the FBI follow up on Tsarnaev?
So far, investigators do not know if the two men ever met.
Surviving bombing suspect Dzhokhar Tsarnaev may have some answers, but he has not talked to investigators since being read his rights a week ago. He remains isolated 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.