According to the filing made Monday, Ibragim Todashev told investigators Tamerlan Tsarnaev participated in a triple slaying in Waltham on Sept. 11, 2011.
In that case, three men were found in an apartment with their necks slit and their bodies reportedly covered with marijuana. One of the victims was a boxer and friend of Tamerlan Tsarnaev.
Todashev, a 27-year-old mixed martial arts fighter, was fatally shot at his Orlando home during a meeting with an FBI agent and two Massachusetts state troopers in May, authorities said. He had turned violent while being question, according to authorities.
The filing is prosecutors' attempt to block Dzhokhar Tsarnaev from getting certain information from authorities, including investigative documents associated with the Waltham slayings.
"The government has already disclosed to Tsarnaev that, according to Todashev, Tamerlan Tsarnaev participated in the Waltham triple homicide," prosecutors wrote.
According to prosecutors, the ongoing investigation into the 2011 slayings is reason not to allow Dzhokhar Tsarnaev access to the documents he's seeking.
"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 of the proceeding," prosecutors wrote.
Prosecutors also said Dzhokhar Tsarnaev is not entitled to the information because his brother's criminal history will be relevant, if at all, only at a possible future sentencing hearing.
A phone message left for a spokeswoman for the U.S. Attorney's Office was not immediately returned Tuesday night. A message left for Dzhokhar Tsarnaev's federal public defender was also not immediately returned.
Authorities allege that Dzhokhar Tsarnaev, 20, and 26-year-old Tamerlan Tsarnaev, ethnic Chechens from Russia, planned and carried out the twin bombings near the finish of the marathon on April 15. Three people were killed and more than 260 were injured.
Dzhokhar Tsarnaev faces 30 federal charges, including using a weapon of mass destruction and 16 other charges that carry the possibility of the death penalty.
Tamerlan Tsarnaev died in a gunbattle with police as authorities closed in on the brothers several days after the bombings.
