It was one of the biggest unsolved crimes in recent Boston memory: three men, killed on Sept. 11, 2011, their throats slit, their bodies reportedly covered with marijuana.

But now, a suspect has been named in the lengthy court documents being filed before the trial of suspected Boston Marathon bomber Dzokhar Tsarnaev: his deceased brother, Tamerlan Tsarnaev.

A document filed in federal court by prosecutors says Tamerlan participated in the triple homicide, apparently based on information provided by a friend of the suspect, Ibragim Todashev.

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The origins of this information are not detailed in the filing, but Todashev was questioned by FBI and state police in Florida in May. He was shot dead by authorities during questioning.

Civil rights groups and Todashev’s father have spent the last five months demanding an explanation as to why Todashev was killed, but have not gotten answers. The Council on American-Islamic Relations released photos of Todashev’s body showing he was shot seven times, once in the head.

Unnamed sources have previously linked Tsarnaev to the triple murder, but this marks a more official link between the two.  

“The government has already disclosed to Tsarnaev, that, according to Todashev, Tamerlan Tsarnaev participated in the Waltham triple homicide,” the document says, on nearly the last page. It is a filing by the government to prevent Dzokhar Tsarnaev’s lawyers from getting certain investigative documents from the government.

The triple murder of Brendan Mess, 25, Erik Weissman, 31, and Raphael Teken, 37, baffled police because it was so violent. All three men were either involved in martial arts or bodybuilding, but were killed violently with few signs of struggle, and little evidence on the scene.

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Tsarnaev’s lawyers are seeking any and all documents that prosecutors have compiled about the triple homicide, as well as government interviews with the Tsarnaevs' extended family and friends. In August, Tsarnaev’s lawyers also filed a motion asking that the conditions of his imprisonment be relaxed -- he is currently being held in solitary confinement at a medical detention facility in western Massachusetts.

In a memo made public this month, Atty. Gen. Eric Holder argued against relaxing Tsarnaev’s prison conditions, saying there was good reason to prevent him from having any outside communications, and disclosing the prisoner had received more than 1,000 pieces of mail since being imprisoned.

Meanwhile, the strange case of Ibragim Todashev continues to agitate civil rights groups. Todashev’s girlfriend has been questioned and one of his friends has been deported.

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alana.semuels@latimes.com

Twitter: @AlanaSemuels