BOSTON » The surviving Boston Marathon bombing suspect, who faces a potential death penalty, is fighting to get access to investigative records that implicate his dead brother in a triple slaying in 2011.

In a filing this week in federal court, prosecutors say they have alerted Dzhokhar Tsarnaev that a friend alleges that Tsarnaev’s older brother, Tamerlan, participated in the killings.

The friend, Ibragim Todashev, was shot to death later 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.