Defense attorneys have called on federal prosecutors to hand over any records they have on suspect Dzhokhar Tsarnaev's older brother, Tamerlan, who authorities contend also played a role in the attacks but died a few days later in a gun battle with police.
He had been monitored by the FBI after traveling to Russia in 2011.
The surviving Tsarnaev, 20, faces the possibility of execution if convicted of planting homemade pressure-cooker bombs at the marathon finish line a year ago, killing three people and injuring 264 in the worst mass-casualty attack on U.S. soil since Sept. 11, 2001.
Defense lawyers last month asked for records on the elder Tsarnaev, arguing they could show their client operated under his "domination and control," which could make him less culpable in the attacks if found guilty and thereby lower his odds of being put to death.
Prosecutors, in turn, last week asked defense attorneys to turn over any evidence they planned to introduce at trial and to notify them by early next month if they plan to introduce expert testimony on Tsarnaev's mental health.
Proof that Tsarnaev was not mentally capable of fully understanding his actions, known as the "insanity defense," could also reduce his odds of being convicted. Federal law requires that prosecutors be given advance notice of such a defense so they may hire mental experts to perform their own evaluation of the defendant.
Tsarnaev, who is not expected in court on Wednesday, was the younger of two ethnic Chechen brothers who lived in the Boston area for about a decade and who federal officials contend left the bombs at the finish line on April 15, 2013.
Three days after the attack, prosecutors contend, the brothers killed a university police officer in an unsuccessful effort to steal his gun as they tried to flee the city. That night, Tamerlan, 26, died in a gun battle with police.
The surviving brother, Dzhokhar, was captured on April 19 and is being held in a prison west of Boston, awaiting trial scheduled to begin in November.