US media reported Federal Bureau of Investigation (FBI) agents killed him in a violent altercation on Wednesday.
The FBI said the man initiated a "violent confrontation" during questioning in Orlando, Florida and was killed, and that an FBI agent suffered "non-life threatening injuries".
The man, identified by the Orlando Sentinel newspaper as Ibragim Todashev (27) was being questioned by the FBI agent, two Massachusetts state troopers and other law enforcement personnel.
Todashev attacked the FBI agent with a knife just as he was about to sign a confession that he had played a role in the triple murder, local media cited investigators as saying.
The triple murder took place on September 11 2011 – the 10-year anniversary of the deadly attacks on the United States – in the Boston suburb of Waltham, according to local media.
All three bodies were found nearly decapitated, covered with marijuana and thousands of dollars in cash.
Local NBC television affiliate WESH said Todashev and Tsarnaev killed Brendan Mess (25) Raphael Teken (37) and Eric Weissman (31) after ripping them off for drugs because they did not want the men to later identify them.
Tsarnaev (26) and his 19-year-old brother Dzhokhar are alleged to have carried out the April 15 bombings at the Boston Marathon, which killed three people and wounded more than 260.
Tamerlan Tsarnaev was killed in a shoot-out with police after the bombings, while his brother was captured and is in custody.
Todashev was not suspected of playing any part in the bombings, NBC reported.
But his friend Khusn Taramiv told WESH that the FBI had expressed interest in him since the bombing.
They had both lived in Boston and were mixed martial arts fighters, Tamariv added that Todashev spoke briefly by phone with Tsarnaev last month.
Taramiv said the FBI interviewed him and Todashev for nearly three hours. – AFP