A Wisconsin prisoner featured in the Netflix series Making A Murderer made a voluntary confession and was properly convicted, a court has heard.
Brendan Dassey was sentenced to life in prison in 2007 in connection with Teresa Halbach's death two years earlier.
He told detectives he helped his uncle, Steven Avery, rape and kill Ms Halbach in the Avery family's Manitowoc County salvage yard.
Avery was sentenced to life in prison in a separate trial.
A federal magistrate judge overturned Dassey's conviction in August, ruling investigators took advantage of the then 16-year-old Dassey's cognitive disabilities and tricked him into confessing with false promises that he would be all right.
The state Department of Justice (DoJ) has appealed, and Dassey remains in prison pending the outcome.
Lawyers for both the DoJ and Dassey presented oral arguments to a three-judge panel at the 7th US Circuit Court of Appeals in Chicago today.
The Milwaukee Journal Sentinel reported that DoJ Deputy Solicitor General Luke Berg told the panel detectives never made Dassey any specific promises.
Judge Ilana Rovner asked whether Dassey, whom the judge described as "extremely suggestible", would not have concluded based on the questioning that he would be able to go home rather than getting arrested.
Mr Berg insisted the investigators acted properly and did not so much as imply promises.
Judge David Hamilton seemed to dispute that, telling Mr Berg that obviously the investigators made vague promises of leniency.
Dassey's lawyer, Laura Nirider, argued the detectives made a "drumbeat of promises" before every major admission in the confession.
Judge Hamilton, however, told her he had watched the entire interrogation and did not think Dassey's will was subverted.
The arguments lasted less than an hour. The panel has no timetable for a decision and it could be months before they rule.
Dassey, now 27, and Avery have contended police framed them.
They believe police went after them to stop a lawsuit Avery had filed demanding millions from Manitowoc County because he spent 18 years in prison for a rape he did not commit.
Avery is pursuing his own appeal.
Their cases gained national attention in 2015 after Netflix aired Making A Murderer, a multi-part documentary examining Ms Halbach's death.
The series spawned widespread conjecture about their innocence.
Authorities who worked on the cases say the series was biased.