US federal prosecutors have said dead Boston Marathon bombing suspect Tamerlan Tsarnaev took part in a 2011 triple murder in a nearby town.

The claim was made by a Chechen immigrant who was himself killed when approached by investigators for questioning.

Ibragim Todashev, 27, told investigators Tsarnaev participated in the murders of three men found stabbed to death in September 2011 in an apartment in Waltham, Massachusetts.

US 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 defence attorneys investigative materials pertaining to his older brother.

The siblings, both ethnic Chechens, are suspected of planting two pressure-cooker bombs that detonated at the finish line of the Boston Marathon on 15 April, killing three people and injuring 264 others.

After allegedly shooting a police officer 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 dead, and 20-year-old Dzhokhar, escaping.

Police later captured the younger Tsarnaev after a day long 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 defence could jeopardise the continuing investigation into the triple homicide.