The alleged involvement of Tamerlan Tsarnaev in the killing in Waltham was disclosed in a government court filing made Monday.
The document says Ibragim Todashev told investigators Tamerlan Tsarnaev participated in the killings. The filing is part of prosecutors' attempt to block Dzhokhar Tsarnaev from getting investigative documents associated with the Sept. 2011 deaths.
Authorities previously said they were investigating possible links between the bombing and the slaying case.
Tamerlan Tsarnaev died following a shootout with police four days after the April 15 twin explosions killed three people and injured more than 260.