Emphasis added:

The new details of what Dzhokhar Tsarnaev has told authorities emerged as the F.B.I. moved forward on Thursday with trying to determine how the brothers were radicalized and the role that Tamerlan’s wife, Katherine Russell, may have played in the plot or in helping the brothers evade the authorities after the attacks.

As part of those efforts, the authorities have sought to determine whether fingerprints and DNA found on bomb fragments were from Ms. Russell. According to two other law enforcement officials, Ms. Russell’s fingerprints and DNA do not match those found on the fragments. All of the law enforcement officials were granted anonymity because they did not want to be identified discussing a continuing investigation.

Federal authorities are skeptical of Ms. Russell’s insistence that she played no role in the attack or in helping the brothers elude the authorities after the F.B.I. released photographs of them. That skepticism has been stoked by Ms. Russell’s decision in recent days to stop cooperating with the authorities.

via Boston Plot Said to Initially Target July 4 - NYTimes.com.