In new court papers, federal prosecutors implicated slain Boston Marathon bomber Tamerlan Tsarnaev in a September 2011 triple homicide in Waltham, Mass. Tsarnaev and an accomplice allegedly slit the throats of three men and covered their bodies with marijuana after a falling out over a drug deal. FBI agents got to know of Tsarnaev's involvement in the murders during an interrogation of fellow co-conspirator Ibragim Todashev at his Orlando apartment in May. A martial arts fighter, Todashev suddenly turned violent upon being questioned about his links to Tsarnaev and was shot dead by a federal agent.

In their filing, prosecutors argued against providing Tamerlan's brother, and surviving Boston Marathon bomber Dzhokar Tsarnaev, access to information, including investigative documents, related to the Waltham triple murder. Dzhokar Tsarnaev "might want access to the information now in order to prepare to defend against it," prosecutors said. They argued that "the government had already disclosed to Tsarnaev that, according to Todashev, Tamerlan Tsarnaev participated in the Waltam triple homicide."

"Any benefit to Tsarnaev of knowing more about the precise 'nature and extent' of his brother's involvement does not outweigh the potential harm of exposing details of an ongoing investigation into an extremely serious crime, especially at this stage in the proceedings," the filing said.

Tsarnaev was charged in federal court in Boston with using a weapon of mass destruction in connection with the April 15 Boston Marathon bombings that killed three people and injured 200 others.