Lawyers for Boston Marathon bombing suspect Dzhokhar Tsarnaev are hoping the FBI has evidence that the defense can use to show his deceased older brother was mostly responsible for the deadly attack.

His attorneys believe that convincing jurors that Tamerlan Tsarnaev was the instigator could sway them to give Dzhokhar's a life sentence instead of the death penalty if he's convicted.

The lawyers asked a judge on Friday to order federal prosecutors to turn over records of FBI contact with Tamerlan, based on information that the agency asked Tamerlan to be an informant on the Chechen and Muslim communities.

The Boston FBI office has denied using the Tsarnaev brothers as sources.

The April 15 bombings killed three people and injured more than 260.

Tamerlan died in a shootout with police.