In a filing this week in federal court, prosecutors say they have alerted Dzhokhar Tsarnaev (joh-HAHR' tsahr-NEYE'-ehv) that a friend alleges that Tsarnaev's older brother, Tamerlan, participated in the killings.
The friend, Ibragim Todashev (IH'-bruh-heem TOH'-duh-shehv), was later shot to death in Florida while being questioned by authorities.
The younger Tsarnaev's lawyers have argued that evidence about his brother's involvement in the triple killings provides "mitigating information" that is critical to his defense.
But prosecutors argue it is an ongoing investigation that could be harmed by such a disclosure.
The Middlesex district attorney's office would not comment on the filing.