Federal prosecutors say they have a witness who is prepared to testify that Boston-Marathon-bombing suspect Dzhokhar Tsarnaev knew his older brother, Tamerlan Tsarnaev, took part in a 2011 triple homicide in the Boston suburb of Waltham. The information regarding the witness was disclosed not by the prosecution but by Tsarnaev's defense team in federal court filings Friday, as they sought discovery access to evidence gathered by the federal government in its case.

The government "disclosed that an identified witness would be prepared to testify" that Dzhokhar knew of his older brother’s involvement, according to a defense filing.

"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 team," it read. "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."

Prosecutors have been successful thus far in thwarting efforts by the defense team to obtain the evidence, but the lawyers argued in the filing on Friday that "even the government has conceded that evidence concerning Tamerlan’s participation in (the) Waltham murders might be relevant if Dzhokhar were aware of it."

According to the Massachusetts medical examiner's report, all three victims were found stabbed in the neck in the exact same manner on Sept. 12, 2011. Ibragim Todashev, a friend of Tamerlan Tsarnaev, reportedly tied the older brother—and possibly himself—to the murders, but was shot to death under undisclosed circumstances while being questioned by the FBI on May 22, 2013. Tamerlan Tsarnaev was killed during a shootout with police three days after the bombings that killed three and injured nearly 300. Dzhokhar Tsarnaev faces the death penalty if convicted on 30 charges in the bombings. He is scheduled to go on trial January 5.