BOSTON (AFP) - Tamerlan Tsarnaev, the older brother in the suspected Boston bombings duo, has been implicated in a triple murder in which the victims were covered in marijuana and their throats slashed.

In a document filed on Monday as part of preparations for the younger brother's trial, Tamerlan was formally linked to the earlier 2011 triple homicide in the Boston suburb of Waltham.

The killings, which took place on Sept 11, 2011 - the 10-year anniversary of the Al-Qaeda attacks on the United States, remain unsolved.

All three bodies were found nearly decapitated, covered with marijuana and thousands of dollars in cash.

Tamerlan, who was killed in a shootout with police after the twin bombings at the Boston Marathon, allegedly carried out the Boston attacks with his then 19-year-old brother Dzhokhar, killing three people and wounding more than 260 others on April 15.

Tamerlan's potential involvement in the Waltham murders was first reported after a Chechen man, investigated for his links to the brothers, was questioned by the FBI.

Ibragim Todashev, 27, was shot dead by agents allegedly acting in self defence after he lunged at them, while being questioned at his Florida home in May.

Investigators said Todashev had been on the verge of signing a confession that he and Tamerlan were involved in the September 2011 murders.

It is not yet clear whether there are any other suspects in the 2011 killings.

US attorney Carmen Ortiz said it would be premature to share documents that relate to Tamerlan's involvement in the triple murder at Dzhokhar's bombings trial.

Information about Tamerlan's criminal past would be relevant only if his brother faces a hearing to determine whether he should be sentenced to death, she said.

Dzhokhar is set to go on trial next year over charges that could carry the death penalty.