Story highlights A source told CNN in May that Tamerlan Tsarnaev was fingered in triple slaying

Prosecutors publicly confirm this in a court document

The allegation came from Ibragim Todashev, who was killed during questioning

For the first time, prosecutors in the Boston Marathon bombing case confirmed that deceased suspect Tamerlan Tsarnaev was accused of participating in a gruesome 2011 triple homicide outside Boston.

A federal law enforcement official told CNN back in May that Tsarnaev's involvement in the killings was revealed in an interview between police and Ibragim Todashev, a Chechen known to him.

But the detail was not confirmed until this week, when prosecutors in the case against Tamerlan Tsarnaev's brother, Dzhokhar Tsarnaev, made reference to it in a court document filed Monday.

In the document, prosecutors argue against a defense request for all documents related to Tamerlan Tsarnaev's participation in the triple slaying.

"In any event, the government has already disclosed to (Dzhokhar) Tsarnaev that, according to Todashev, Tamerlan Tsarnaev participated in the Waltham triple homicide," the document states.

Todashev died during the interview with authorities, which occurred at Todashev's house. Todashev attacked an FBI agent, who shot him dead, a federal law enforcement official with direct knowledge of the case told CNN.