If their motion is granted, it could hamper the government's efforts to seize Lay's remaining assets, reports The Washington Post.
Legal experts told the Post the system does not like to punish dead people or where one has not had a chance to appeal. Lay was convicted in May on fraud and conspiracy charges but he died last month before he could appeal or be sentenced.
His lawyers say the court should enter an order vacating his conviction and dismiss the indictment, the report said.
One legal expert told the Post said the conviction most likely will be struck down but that civil cases filed by shareholders may still allow the government to recover some of the assets.
A U.S. Justice Department spokesman was quoted as saying the agency will pursue "all available legal remedies to reclaim for victims the proceeds of crimes committed by Ken Lay."