(CNN) Details about a gruesome triple homicide at a Massachusetts residence in 2011 that a friend of Boston Marathon bomber Tamerlan Tsarnaev said they took part in were revealed this week in federal court documents.

The documents were filed Wednesday in connection with a pending appeal by Dzhokhar Tsarnaev , Tamerlan's younger brother and Boston bombing co-conspirator, of his conviction and death sentence. Oral arguments are scheduled to begin December 12.

Tamerlan Tsarnaev's friend, Ibragim Todashev, who was shot and killed during the 2013 interview with law enforcement, described the killings, which occurred about two years before the bombings, in the new court documents.

While being questioned on May 21, 2013 — about a month after the Boston Marathon attack — Todashev said he and Tsarnaev bound, beat and slit the throats of three young men in the Massachusetts city of Waltham, according to a heavily redacted FBI affidavit filed Wednesday.

"He said that he and Tamerlan had agreed initially just to rob the victims," the FBI special agent, whose name is blacked out, says in the affidavit.

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