(Reuters) - 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.
U.S. media had previously reported that one of the victims was a close friend of Tsarnaev and authorities believe the killings were drug related.
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.
Dzhokhar Tsarnaev's lawyers are seeking to force the government to share with defense attorneys investigative materials pertaining to his deceased older brother, Tamerlan.
The two siblings, both ethnic Chechens, are suspected of planting two pressure-cooker bombs that detonated at the finish line of the Boston Marathon on April 15, killing three people and injuring 264 others.
After allegedly shooting a police officer to death in an ambush three days later, the pair went on to engage in a late-night gun battle with police in nearby Watertown that ended with Tamerlan Tsarnaev, 26, dead, and 20-year-old brother, Dzhokhar, escaping.
Police later captured the younger Tsarnaev after a daylong manhunt in which most of the Boston area was placed on a security lockdown.
Dzhokhar has since pleaded not guilty to charges related to the bombing and the policeman's slaying.
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.
Reporting by Brendan O'Brien in Milwaukee and Steve Gorman in Los Angeles; Editing by Lisa Shumaker