The federal public defenders assigned to represent Boston marathon bombing suspect Dzhokhar Tsarnaev today filed a brief in federal court, stating, among other things, that they wrote a letter to the FBI seeking information about allegations that the bureau asked Tamerlan Tsarnaev to become an informant. Below I've copied the relevant portion of the motion. It's text from a February 28, 2014 letter the defense team sent to the federal government. [Emphasis mine.]

We seek this information based on our belief that these contacts were among the precipitating events for Tamerlan’s actions during the week of April 15, 2014, and thus material to the defense case in mitigation. We base this on information from our client’s family and other sources that the FBI made more than one visit to talk with Anzor, Zubeidat and Tamerlan, questioned Tamerlan about his internet searches, and asked him to be an informant, reporting on the Chechen and Muslim community. We further have reason to believe that Tamerlan misinterpreted the visits and discussions with the FBI as pressure and that they amounted to a stressor that increased his paranoia and distress. We do not suggest that these contacts are to be blamed and have no evidence to suggest that they were improper, but rather view them as an important part of the story of Tamerlan’s decline. Since Tamerlan is dead, the government is the source of corroboration that these visits did in fact occur and of what was said during them.

In response to this letter, the government replied on March 14 that it has "no evidence" the FBI sought out the elder brother as an informant. Tsarnaev's attorneys nonetheless push harder for relevant information, writing to the judge:

No one other than Tamerlan and the agents involved were actual witnesses to what transpired, and absent verification from the government itself, the defense will have no direct evidence to establish the reasons for these contacts, or their number, nature, and content. This information is critical to explain Tamerlan’s changes in behavior and functioning, and should be disclosed.

The brief also includes information suggesting that the defense is pursuing a strategy related to Tamerlan Tsarnaev's influence on his younger brother, the defendant. Specifically, it seems as if the attorneys may argue that Tamerlan's alleged involvement in a gruesome triple murder in Waltham, Massachusetts on the tenth anniversary of the 9/11 attacks may have frightened his little brother. The attorneys write:

As for the Waltham crimes themselves, it should be added that Tamerlan’s having committed a gruesome triple murder — and having included a “close friend” among the victims — would powerfully support the inference that Dzhokhar experienced his older brother as an all-powerful force who could not be ignored or disobeyed. Since Todashev was shot and killed by FBI agents while confessing to his role in the Waltham murders, the defense has no remaining source for what Todashev knew other than the government. The Todashev 302s and any other memorialization or records of his May, 2013 interviews are Brady material and should be disclosed.

You can read the entire motion here.