Prosecutors on Monday appealed a high court decision ordering the retrial of a hospital worker who served 12 years behind bars after she was convicted of killing an elderly patient in 2003 by removing his respirator.

Mika Nishiyama, 37, was found guilty in 2005 by the Otsu District Court of causing the death of a 72-year-old male patient in a Shiga Prefecture hospital where she worked as a nursing assistant.

The high court and the Supreme Court later upheld the conviction and Nishiyama completed her prison term last August.

But the Osaka High Court said last Wednesday that the patient may have died a natural death and not necessarily due to loss of oxygen supply.

Yasuo Nakazawa of the Osaka High Public Prosecutor's Office said in a statement, "We will refrain from commenting on details at this stage."

Prosecutors can appeal a high court ruling only on grounds of a constitutional violation or a conflict with judicial precedent.

The focal point of the trial was the credibility of Nishiyama's confession. She initially admitted to investigators that she removed the respirator and killed the patient but pleaded not guilty in subsequent court proceedings, claiming that interrogators induced her to make a false confession.

Nishiyama's initial application for a retrial, filed in 2010, was rejected and the latest process is part of her second try.

In September 2015, the district court dismissed the second request. But the high court pointed to the possibility of a natural death, based on medical documents and judicial autopsy data submitted as new evidence by Nishiyama's defense team.

The high court also said in its decision that "reasonable doubt" has emerged to judge Nishiyama a criminal, adding her initial confession made to police officers may have been untrue.