(CBS News) There is new focus in the Boston Marathon bombing investigation on the widow of suspect Tamerlan Tsarnaev. Investigators have identified female DNA on a fragment of one of the bombs used in Boston. It's not clear what that means -- the DNA could have come from an injured spectator, or perhaps from a clerk who sold the materials used in the bombs, according to law enforcement sources. But, there's also a chance, it could lead the FBI to a female accomplice.
U.S. officials tell CBS News' Bob Orr that FBI agents on Monday visited the Rhode Island family home of Katherine Russell, the widow of accused bomber Tamerlan Tsarnaev.
Sources say agents were there to collect a DNA sample from Russell to compare it against the female DNA found on a bomb fragment at the blast scene. Investigators say a lab test could help them determine if Russell ever had contact with the device.
Investigators say Russell is not a suspect and hasn't been charged in the plot. Her attorney has said she is fully cooperating with the investigation.
If the female DNA found on the bomb was Tamerlan Tsarnaev's widow's, CBS News senior correspondent John Miller, a former assistant FBI director, said on "CBS This Morning," it becomes "a critical moment" in the investigation. "(Law enforcement's) relationship with her may change," Miller said. "And the question is what do you do with that? She's already represented by a lawyer. It certainly becomes a circumstantial that says she may have handled the device or parts of the device and that opens the door she may have known."
Of particular interest to investigators may also be where the bombs were tested. Miller explained, "What they found in the apartment was black powder and parts, but not the work bench you would need to have to make these bombs. They are still looking for that another place. One way is they recovered a GPS device belonging to these guys after the big shootout in the carjacked vehicle. That could tell them some things, depending on what they saved and what they got rid of. It may have trips way outside of the city where you could have tested the bomb or take you to a location where they could have made that bomb. And even if they wiped everything out, they're still going back through the systems of his cell phone carrier, 60 days, 30 days, 90 days, is there a trip to the Berkshires, to the woods, to some remote area because, even if you follow the instructions perfectly, you don't put those two devices down on the marathon without having taken them somewhere first and tested them out to make sure, 'Does my main charge function with this initiator,' and 'does my remote control make it go?'"
Watch John Miller's full analysis in the video below.
At the same time, FBI investigators are in Russia and Dagestan, trying to find out if Tamerlan Tsarnaev, and his younger brother Dhzokhar Tsarnaev, had any foreign terror connections, U.S. officials tell CBS News' Bob Orr.
Of particular interest, a Russian-born, Canadian militant named William Plotnikov, according to law enforcement sources. Like Tamerlan 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 Plotnikov and Tsarnaev ever met. And there is no evidence to date that the Tsarnaev brothers had any connections to any wider conspiracy.
Meanwhile, Dzhokhar Tsarnaev remains in a federal medical center lockup outside of Boston. He has not answered any questions since being read his rights a week ago.
Dzhokhar Tsarnaev's defense team has added veteran death penalty opponent in Judy Clarke. She has represented several high profile defendants, including the unabomber Ted Kaczynski and Jared Loughner, the gunman who shot Rep. Gabrielle Giffords in Tucson. Both of those defendants avoided the death penalty. Legal sources tell CBS News it would be no surprise if DkzhokarTsarnaev seeks some kind of cooperation deal to do the same.
Watch Bob Orr's full report in the video above.