Prosecutors say court documents show Tamerlan Tsarnaev played a part in a triple slaying in Waltham, Mass., in 2011. Three men were executed following a botched drug deal, according to the newly released documents.
Brendan Mess, Erik Weissman, and Raphael Teken were all killed by sharp wounds to their neck in 2011. Their bodies were found in a Waltham residence covered with marijuana. Investigators also found $5,000 in cash nearby one of the victims.
Mess had been roommates with Tamerlan Tsarnaev. Both men boxed and trained in martial arts together.
Investigators told ABC News that cell phone records show the Tsarnaev brothers were in the area the same day of the murders.
Ibragim Todashev, the man who identified Tamerlan as one of the assassins, was later fatally shot while being questioned by authorities about the case in his Florida home in May.
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.
The crime remains unsolved but authorities said crime scene forensic evidence links Tamerlan to the investigation.
Tamerlan died in a gun battle with police as they closed in on him and his younger brother, days after the deadly bombings that killed three people and injured approximately 260.
According to his death certificate, Tamerlan died of gunshot wounds and blunt trauma to his head and torso.
Police have said he ran out of ammunition before his younger brother dragged his body under a car while fleeing the scene.
Dzhokhar Tsarnaev, 20, was captured hiding in a tarp-covered boat the day after the shootout. He is now fighting to get access to investigate the records that implicate his brother.
The 20-year-old faces 30 federal charges in relation to the Boston Marathon bombing. His lawyers have argued that the evidence about his brother's involvement in the triple killings provides "mitigating information" that is critical to his defense.
Prosecutors argue it is an ongoing investigation that could be harmed by such a disclosure.
State investigators who oversee the homicide case said they have an active and ongoing investigation into the triple murder.
ABC News and The Associated Press contributed to this report.