Samoa's Office of the Attorney General is appealing against a judge's decision to overturn the assessors' guilty verdict on Jonathan Patrick Crichton.
An assessors' panel had found Mr Crichton guilty of the murder of New Zealander Hans Dalton.
Mr Dalton, who was mentally ill, was found dead, head down in a drum of water at Tafaigata Prison on Boxing Day in 2012.
Talamua online quotes the Attorney General Aumua Ming Leung Wai saying an appeal has been filed.
Mr Crichton originally admitted killing Mr Dalton but later said that he was intimidated by the investigating police officer.
Four assessors were unanimous in their guilty verdict but Justice Vaai over ruled them, saying the prosecution had not proven beyond reasonable doubt it was the accused who placed Mr Dalton in the drum.
Mr Dalton's family is now suing the Ministries of Police and Health.