When Tsarnaev's case begins, his lawyers are likely to pin their hopes and the bombings themselves on his older brother, Tamerlan: an amateur boxer, college student, husband and father who also followed radical Islam and was named by a friend as a participant in a grisly 2011 triple slaying.
"He was the eldest one and he, in many ways, was the role model for his sisters and his brother," said Elmirza Khozhugov, the former husband of Tamerlan's sister, Ailina.
Three people were killed and more than 260 were injured when two homemade pressure-cooker bombs exploded near the finish line of the iconic race on April 15, 2013.
Tamerlan Tsarnaev, 26, died days after the bombings following a gun battle with police. Dzhokhar, then 19, was later found hiding in a boat parked in a backyard. Jury selection in his federal death penalty trial is entering its second month.
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If a jury convicts Dzhokhar, its decision on whether to give him life in prison or sentence him to death could depend "on the extent to which it views Tamerlan Tsarnaev as having induced or coerced his young brother" to help commit the crimes, the defense argued in a court filing.
About a decade before the attack, their parents, ethnic Chechens, had moved the family to the US from the volatile Dagestan region of Russia after living in the former Soviet republic of Kyrgyzstan. Their father, Anzor Tsarnaev, told The Associated Press they emigrated in part to escape discrimination.