Mystery surrounds the death of US student Otto Warmbier after his June release from North Korea as a coroner’s report reveals he was “well-developed” and “well-nourished”.

The Hamilton County Coroner’s report has listed the cause of death as brain damage from a lack of oxygen from an unknown injury “more than a year prior to death”.

The report, which was the result of an external examination only as Otto’s parents had declined a full autopsy, determined the manner of death as “undetermined”, but gave details which seem at odds with his parents’ account detailed in an interview with Fox News on Wednesday (AEST).

The coroner also found Otto’s overall condition to be “well-developed, well-nourished”, noting only there had been a collection of scars on his body.

Fred and Cindy Warmbier said their son’s death was the result of torture by North Korea, and that he had experienced “inhuman” suffering.

“But now we see North Korea claiming to be a victim and that the world is picking on them and we’re here to tell you: North Korea is not a victim. They’re terrorists,” Mr Warmbier said on Fox and Friends. “They kidnapped Otto. They tortured him. They intentionally injured him. They are not victims.”

Mr Warmbier said the first time they saw Otto after he arrived back in the US, their son was making a “howling involuntary, inhuman sound” and that his mouth looked as if  “someone had taken a pair of pliers and rearranged his bottom teeth”.

They added that Otto was blind and deaf and that he was “jerking violently” on a stretcher on his return to the US.

“They destroyed him,” Mrs Warmbier said.

But the report, obtained by PEOPLE magazine and performed by a Cincinnati coroner Dr Gretel Stephens, concluded that Otto’s nose and ears showed “no remarkable alteration” and that “the teeth are natural and in good repair”.

At least half the scars on his body were pale, the report noted, indicating they were older, and that there was some evidence of medical therapy, such as a catheter and a newer scar consistent with that of a tracheostomy.

 Otto Warmbier, 22, died on June 13, only days after returning from North Korea with severe brain damage after having been “brutalized and terrorised by the  regime,” according to his family at the time.

University of Cincinnati Medical Center doctors found him to have suffered “extensive loss of brain tissue in all regions of the brain” and described his  condition at the time as “unresponsive wakefulness”.

“We have no certain or verifiable knowledge of the cause or circumstances of his neurological injury. This pattern of brain injury however is usually seen as a result of cardiopulmonary arrest where the blood supply to the brain is inadequate … resulting in the death of brain tissue,” medical centre doctors said at the time.

Otto had been arrested and sentenced to 15 years hard labour after travelling to North Korea in January 2016 and allegedly stealing a propaganda poster.

North Korean officials have consistently denied any mistreatment, maintaining that Otto had contracted botulism and went into a coma after taking a sleeping pill.

PEOPLE reported that the Warmbiers did return their request for comment.