The outlines of a possible defence have come into focus after it emerged Tsarnaev's lawyers were trying to get access to investigative records implicating his now-dead brother in a grisly triple murder committed in 2011.
In court papers, federal prosecutors acknowledged publicly for the first time that a friend of Tamerlan Tsarnaev told investigators he participated in the unsolved killings of three men who were found in an apartment in Waltham, Massachusetts, with their throats slit and marijuana sprinkled over their bodies.
The younger Tsarnaev's lawyers argued in court papers any evidence of Tamerlan's involvement was "mitigating information" that was critical as they prepared Dzhokhar's defence. They asked a judge to force prosecutors to turn over the records.
Dzhokhar Tsarnaev, 20, faces 30 federal charges, including using a weapon of mass destruction, over the twin bombings April 15 that killed three people and injured more than 260. Tamerlan Tsarnaev, 26, died in a gun battle with police days later.
The government is still deciding whether to pursue the death penalty for the attack, which investigators say was retaliation for the US wars in Muslim lands.