A panel of judges at the Dirksen Federal Building heard arguments for and against Brendan Dassey's release. The 27-year-old man was sentenced in 2007 to life in prison for the 2005 murder of photographer Teresa Halbach.
Dassey had told officers he helped his uncle, Steven Avery to rape, kill and mutilate Halbach in the Avery family's Manitowoc County salvage yard.
A federal magistrate overturned Dassey's conviction in August 2016 and ordered his release, after finding investigators tricked him into a confession.
The Wisconsin attorney general tried to convince the panel of judges Tuesday that Dassey's confession was voluntary and that detectives had done nothing wrong.
Dassey's attorneys said their client was easily compelled to confess because he was too young and his IQ was too low to understand that comments from interrogators like, "the truth will set you free," shouldn't be taken literally.
"I am hopeful on behalf of the state that the confession will not be overturned and that this decision will affirm what it is that the state courts of Wisconsin have consistently held in this case," Ken Kratz, former prosecutor in the Dassey case.
As of 11:30 a.m. Tuesday, Dassey's attorneys had not yet come down to the lobby of the federal building to give comment.
Dassey will remain in jail pending the outcome of the appeal.
Avery was sentenced to life in prison in a separate trial. He's pursuing his own appeal.
The Associated Press contributed to this report.