Lawyers challenging the death sentence of Boston Marathon bomber Dzhokhar Tsarnaev are seeking recordings of law enforcement interviews with a triple-homicide suspect who was killed in a 2013 confrontation with authorities.

In a document submitted Tuesday with the US Court of Appeals for the First Circuit in Boston, attorneys for Tsarnaev requested permission to file their motion for “reports and recordings of interviews of Ibragim Todashev’’ under seal.

The filing did not indicate why Tsarnaev’s appellate team wants to review the Todashev materials.

Todashev was a 27-year-old mixed-martial-arts fighter when he allegedly confessed to Massachusetts State Police and FBI agents in May 2013 that he and Tsarnaev’s older brother, Tamerlan, had participated in the slayings of three men in Waltham in 2011.

Shortly after the confession, Todashev was fatally shot by an FBI agent after Todashev allegedly charged at the agent and a state trooper with a metal broomstick.

No one has been charged in connection with the Waltham murders.

Dzhokhar and Tamerlan Tsarnaev set off the bombs that killed three people and wounded hundreds more during the Boston Marathon in April 2013. Tamerlan Tsarnaev was killed during a clash with police in Watertown days after the blasts.

The brothers, who lived in Cambridge, also killed an MIT police officer while they were on the run.

Dzhokhar was convicted for his role in the Marathon attack and sentenced to death in 2015. His appeal is pending.

The 24-year-old is being held at a federal supermax prison in Colorado.

Travis Andersen can be reached at travis.andersen@globe.com. Follow him on Twitter @TAGlobe.