Similarly, Aitan D. Goelman, who was part of the legal team that prosecuted Oklahoma City bombing figures Timothy McVeigh and Terry Nichols, said the defense may be looking to minimize the younger brother's role in the bombing.
"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.
Investigators have given no motive for the 2011 slayings. One victim was a boxer and friend of Tamerlan Tsarnaev's.
Friends of those killed have said they gave Tsarnaev's name to investigators at the time. That has raised questions of whether authorities missed an opportunity to prevent a bigger tragedy.
Federal prosecutors said in court papers that Ibragim Todashev, another friend of Tamerlan's, told authorities that Tamerlan took part in the killings. Todashev was shot to death in Florida in May by authorities while being questioned.
Prosecutors argued that turning over the records would damage the investigation into the killings.
Smith reported from Providence, R.I. Associated Press writer Pete Yost in Washington contributed to this report.