(CBS)-- CBS News has learned the DNA and fingerprints on bomb fragments found at the Boston bombing scene do not match those of Tamerlan Tsarnaev's widow, Katherine Russell.
But Russell is still under scrutiny due to her proximity to the suspected Boston bomber, according to CBS News senior correspondent John Miller, a former assistant FBI director.
"I think they're very skeptical about (her story)," Miller said on "CBS This Morning." "But again, 'skeptical' isn't proof and that's why they're trying to work through that and I think that's why her lawyers are slowing that down."
Investigators are looking at a phone call between Russell and Tamerlan Tsarnaev just hours after the FBI released a picture of him. Miller said the "nature of that call" is of particular interest.
"Did (Tamerlan and Dzhokhar Tsarnaev) disclose to her that they were the bombers? Did they say they were going on the run? Did they give her instructions to assist with any escape?" Miller said. "But there's no known tape of that call, so really the account they're going to have to get of that since Tamerlan is dead is hers."
What continues to puzzle the FBI about Russell is what she knew and when, if anything, Miller said. "What they really want to know from her is, you know, here's a woman who dated Tamerlan, who converted to Islam after they were married, who ... lived in the same apartment -- not a big apartment -- where bombs were being constructed and pressure cookers were being bought two or three at a time," Miller said. "They want to know, 'Is it possible all of this was going on concealed from you or did you know it the whole time?'"
Cooperative with police since the initial stages of the investigation, Russell has begun to communicate with law enforcement through lawyers.
Miller explained, "The other day they showed up at her family's place where she's staying now in Rhode Island with a court order to take DNA samples, hair samples and so on to compare with what they found on the bomb.
"From that point on what's being put out is that she's no longer cooperating," Miller said. "I think what we're actually seeing is lawyers doing their job, which is she retained counsel and it was very clear she was developing into a potential target of this investigation and they basically said, 'If you want anything -- it's not that we're not cooperating -- but bring it to us and we'll send you back an answer.'"