The team prosecuting accused Boston Marathon bomber Dzhokhar Tsarnaev asked defense attorneys on Friday to notify them by next month if they plan to make the 20-year-old defendant's mental abilities an issue at trial.
In a filing in U.S. District Court in Boston, prosecutors asked a judge to require the defense to notify them by May 7 if they plan "to introduce expert evidence relating to a mental disease or defect or any other mental condition of the defendant which bears on either the issue of guilt or the issue of punishment."
Attorneys for Tsarnaev, who faces the threat of death if convicted of the April 2013 attack that killed three people and injured 264, could argue that the defendant is not wholly responsible for his actions or not deserving of execution, legal experts said.
"There aren't a lot of directions you can go as a defense attorney," said Walter Price, a former federal prosecutor. "One of them is always mental and if you think you have the potential to argue that the defendant had diminished mental capacity or insanity, you have to prove it."
Federal court rules require defense attorneys to notify prosecutors of their plans to mount such a defense to give the prosecution time to conduct its own expert review, legal experts said.
Tsarnaev was the younger of a pair of ethnic Chechen brothers who had lived in the Boston area for about a decade who federal officials contend left a pair of homemade pressure-cooker bombs at the race's finish line on April 15, 2013.
Three days after the attack, prosecutors contend, the pair killed a university police officer in an unsuccessful effort to steal his gun as they tried to flee the city. That night older brother Tamerlan Tsarnaev, 26, died in a gunbattle with police.
The surviving brother was captured on April 19 and is being held in a prison west of Boston, awaiting a trial scheduled to begin in November.
A defense attorney for Tsarnaev did not immediately respond to a request for comment on Friday.
But another former prosecutor, attorney Thomas Peisch, said it was highly likely that Tsarnaev's defense team would argue during the trial that mental disabilities reduced his blame in the bombings.
"It's almost unthinkable that the defense won't try to interject diminished capacity at some point," Peisch, a partner at Conn Kavanaugh, said.