The outlines of a possible defence came into focus this week when it was learned that Tsarnaev's attorneys are trying to get access to investigative records implicating the now-dead brother in a grisly triple slaying committed in 2011.
In court papers Monday, federal prosecutors acknowledged publicly for the first time that a friend of Tamerlan Tsarnaev told investigators that Tamerlan participated in the unsolved killings of three men who were found in a Waltham apartment with their throats slit, marijuana sprinkled over their bodies.
Dzhokhar Tsarnaev, 20, faces 30 federal charges, including using a weapon of mass destruction, in the twin bombings April 15 that killed three people and injured more than 260. Tamerlan Tsarnaev, 26, died in a gunbattle 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.
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"If I was a defence attorney and was seeking perhaps to draw attention to the influence the older brother had in planning the bombing, I would use his involvement in other crimes to show that he was likely the main perpetrator in the Boston bombing," Dieter said.
"I would take the position that my client, the younger brother, was strongly influenced by his older brother, and even if he is culpable, the death penalty is too extreme in this case."
"I think the most likely reason is that if they are arguing some kind of mitigation theory, that the older brother was a monster and the younger brother was under his sway or intimidated or dominated by him," he said.
Miriam Conrad, Tsarnaev's public defender, had no comment.