CHICAGO (AP) — Attorneys for a Wisconsin inmate featured in the popular “Making a Murderer” series say they’re “profoundly disappointed” that an appeals court has overturned a ruling that could have set him free.

The 7th U.S. Circuit Court of Appeals ruled Friday that police properly obtained Brendan Dassey’s confession and he should remain behind bars. The judges were sharply divided, voting 4 to 3 that Dassey’s confession wasn’t coerced, as a lower court had ruled.

Dassey was sentenced to life in prison in 2007 after he told detectives he helped his uncle, Steven Avery, rape and kill photographer Teresa Halbach. A key argument from his attorneys was that mental limitations made him vulnerable to leading questions from detectives.

His attorneys, Laura Nirider and Steven Drizin, say they’ll ask the U.S. Supreme Court to review the case. They say the 7th Circuit judges ignored the fundamental idea that false confessions often result when children or the mentally vulnerable are interrogated.