A prosecution witness is willing to testify Boston Marathon bombing suspect Dzhokhar Tsarnaev knew his older brother, Tamerlan, was involved in an unsolved triple killing in 2011, according to a federal court filing Friday by Tsarnaev's defense team, the Associated Press reported. Tsarnaev's lawyers want access to evidence that could link Tamerlan to the murder to highlight his influence on his younger brother.

"Simply put, information and evidence tending to show that Tamerlan Tsarnaev, who was killed a few days after the attack in a shootout with police in Watertown, Mass., participated in a triple homicide in 2011, and information depicting the brutality of those murders, is critical to the defense case in mitigation. Such evidence would tend to corroborate Tamerlan's dominant role in the charged offenses and would place the brothers' respective personal characteristics and relative culpability into stark relief," reads the filing from Tsarnaev's lawyers.

The killings occurred Sept. 11, 2011, in Waltham, Mass. Tamerlan Tsarnaev was friends with Brendan Mess, one of the victims, but was never questioned during the investigation despite reports mutual friends were suspicious of the older Tsarnaev, Yahoo! News reported. A previous filing reported Ibragim Todashev, a friend of Tsarnaev who was shot and killed by an FBI agent during an investigation into the 2011 killing, was set to sign a confession admitting his and Tamerlan Tsarnaev's role in the triple homicide.

It is believed by legal experts Tsarnaev's lawyers are trying to develop a defense the younger Tsarnaev was forced into his role in the Boston bombing and his brother was the mastermind of the attack which killed three people and injured more than 250 others. Tsarnaev pleaded not guilty and a status hearing for the case is scheduled for Oct. 20. The two sides will release their initial lists of witnesses and federal prosecutors have requested Tsarnaev's appearance during the hearing but no decision has yet been made.