Lawyers for the accused Boston Marathon bomber on Friday asked a judge to order U.S. prosecutors to hand over more information, including surveillance data, on his late older brother in order to assess the relative blame of each man in the attack.
Dzohkhar Tsarnaev, who is charged with killing three people and injuring 264 with homemade bombs at the April 15, 2013, marathon and shooting dead a university police officer a few days later, faces the threat of execution if convicted of the worst mass-casualty attack on U.S. soil since Sept. 11, 2001.
Defense attorneys said any evidence that suggests older brother Tamerlan Tsarnaev, who died after a gunfight with police while the pair were preparing to flee Boston a few days after the attack, could boost the 20-year-old surviving Tsarnaev's chances of avoiding the death penalty if convicted.
"Any evidence tending to show that Tamerlan supplied the motivation, planning, and ideology behind the Boston Marathon attack, and that his younger brother acted under his domination and control, is 'material,'" defense attorneys said in one of a series of a filings in U.S. District Court in Boston.
Noting that a Congressional report released on Wednesday showed that U.S. investigated the elder Tsarnaev after a 2011 tip from Russian authorities that he may have become radicalized, defense attorneys asked for any classified information gathered on the elder Tsarnaev.
"Evidence that shows Tamerlan to have had a substantially longer and deeper engagement than his younger brother with extremist and violent ideology is mitigating for the light that it sheds on their relative culpability," defense lawyers wrote.
Prosecutors have said that they have turned over reams of evidence to Tsarnaev's attorneys and that the follow-up requests are too broad to fill.
The Tsarnaev family immigrated to the United States from Russia's restive Chechnya region about 10 years before the attack, and were granted asylum before taking residence in a small apartment in Cambridge, Massachusetts, just outside Boston.
Defense lawyers wrote that since the family had been evaluated for and granted asylum, the U.S. government would have far more background on them than it would for a typical family of native-born Americans.
The surviving Tsarnaev is being held in a jail west of Boston awaiting a trial scheduled to begin in November.
The bombs that ripped through the crowded finish line of Boston's best-attended sporting event killed three spectators: Martin Richard, 8, Krystle Campbell, 29, and Chinese national Lu Lingzi, 23. Prosecutors charge that the Tsarnaev brothers shot dead a fourth man, 27-year-old Massachusetts Institute of Technology campus police officer Sean Collier, a few days later, in a failed attempt to steal his gun as they tried to flee the city.