BOSTON – Slain Boston Marathon bombing suspect Tamerlan Tsarnaev took part in a 2011 triple homicide in a nearby town, according to a Chechen immigrant who was himself killed when approached by investigators for questioning , federal prosecutors said on Monday in newly filed court papers.

Ibragim Todashev, 27, who has been identified as an acquaintance of Tsarnaev from their days as mixed martial-arts fighters in Boston, told investigators Tsarnaev participated in the murders of three men found stabbed to death in September 2011 in an apartment in Waltham, Massachusetts, according to the filing.

The FBI has said Todashev was shot and killed by a federal agent about a month after the marathon bombings when he suddenly turned violent while being questioned about possible links to Tsarnaev.

The latest disclosure about Tsarnaev came in a 23-page brief arguing against a motion by lawyers for his younger brother, Dzhokhar Tsarnaev, who is in custody on federal charges related to the marathon bombing that carry the death penalty.

Lawyers for the younger Tsarnaev have argued that information about the investigation of his older brother’s possible role in the Waltham murders might be a mitigating factor in the government’s case against Dzhokhar. But prosecutors said disclosure of the materials sought by his defense could jeopardize the continuing investigation into the triple homicide.