BOSTON (AP) — A prosecution witness against Boston Marathon bombing suspect Dzhokhar Tsarnaev is prepared to testify that Tsarnaev knew his older brother was involved in a 2011 triple slaying, according to a filing by attorneys for the surviving brother.

The defense seeks several items from prosecutors, including any evidence linking Tamerlan’ Tsarnaev to the gruesome slayings in Waltham, where the bodies of three men were found with their throats slit and sprinkled with marijuana. The information was disclosed in the filing by the defense late Friday.

“Simply put, information and evidence tending to show that Tamerlan Tsarnaev participated in a triple homicide in 2011, and information depicting the brutality of those murders, is critical to the defense case in mitigation,” lawyers said in their filing. “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.”

Tsarnaev has pleaded not guilty in the 2013 bombings that killed three people and injured about 260 others. Legal experts say his lawyers may try to argue he fell under the murderous influence of his late brother.

Defense lawyers said that “even the government has conceded” that evidence concerning the older brother’s participation in the Waltham killings might be relevant if Dzhokhar Tsarnaev was aware of it.

Prosecutors have previously declined to provide information on the Waltham case, saying it could jeopardize the investigation.

In court papers filed in October 2013, federal prosecutors acknowledged that a friend of Tamerlan Tsarnaev told investigators that Tamerlan participated in those unsolved killings. That man, Ibragim Todashev, was shot to death by authorities while being questioned in Florida.

Tamerlan Tsarnaev, 26, died in a gunbattle with police a few days after the deadly bombing.