Feds: Info Boston Marathon Bombing Suspect 'Participated' in 2011 Triple Murder

PHOTO: Tamerlan Tsarnaev accepts the trophy for winning the 2010 New England Golden Gloves Championship in Lowell, Mass. FBI officials released this photo of Dzhokhar Tsarnaev.Courtesy The Sun of Lowell, Mass| FBI/AP Photo
Tamerlan Tsarnaev, left, and Dzhokhar Tsarnaev, right, are suspected in connection with the April 15 Boston Marathon bombings.

Federal prosecutors have disclosed publicly for the first time that they have information indicating deceased Boston Marathon bombing suspect Tamerlan Tsarnaev "participated" in a September 2011 unsolved triple homicide.

The revelation comes in a court document filed in the case against surviving bombing suspect, Dzhokhar Tsarnaev, Tamerlan's younger brother. In the document, prosecutors note that they told Dzhokhar his dead brother was implicated in the triple murder by a friend, Ibragim Todashev.

Todashev was shot dead in his Florida apartment in May during an interview with the FBI and Massachusetts State Police. Sources with direct knowledge of the incident told ABC News shortly after the shooting that Todashev suddenly attacked the officers before he was "about to sign a statement" admitting to his own role in the triple murder.

According to the new court filing, before his death Todashev told investigators Tamerlan "participated" in the triple homicide in Waltham, Mass., in which three bodies were discovered naked with marijuana strewn across them on the tenth anniversary of the Sept. 11, 2001 terror attacks. The new court document did not elaborate on Tamerlan's alleged involvement.

Law enforcement officials told ABC News days before Todashev's death that there was "mounting evidence," bolstered by "forensic hits," that pointed to the possible involvement of both Tamerlan and Dzhokhar in the triple murder. One of the young men killed, Brendan Mess, had once been a roommate and martial arts colleague of Tamerlan's, officials said then. At the time, the officials cautioned that DNA testing was not complete and it was too early to bring an indictment against Dhzokhar.

State investigators who oversee the homicide case said they have an active and ongoing investigation into the triple murder.

READ MORE: Graphic, Perplexing Triple Murder on a Dead-End Street

In July Dzhokhar Tsarnaev smiled in court and pleaded not guilty to all 30 charges against him related to the Boston Marathon bombing. The attack, in which two bombs housed in pressure cookers detonated within seconds of each other near the finish line of the Boston Marathon in April, killed three people, including an 8-year-old boy, and injured more than 260 others.

After a massive manhunt that shut down much of Boston, law enforcement caught up to the brothers days later and Tamerlan was killed in a shootout. Dzhokhar was wounded but survived. He was later discovered bloody and hiding in a boat, having allegedly scrawled anti-American messages on the inner wall.

FULL COVERAGE: Boston Marathon Bombing