Mystery surrounds the death of Otto Warmbier after a coroner said his body showed few signs of torture.

The 22-year-old American university student was released by North Korea after reportedly going into a coma shortly after he was sentenced to 15 years hard labour in March 2016. He later died in a US hospital after suffering a serious brain injury.

Speaking to Fox and Friends, Mr Warmbier’s father accused the secretive state of torture. “It looked like someone had taken a pair of pliers and rearranged his bottom teeth,” Fred Warmbier said.

But Lakshmi Sammarco, a medical examiner in Mr Warmbier’s home state of Ohio, said although the student had died from a lack of oxygen and blood to the brain, she could not explain what had caused the condition.

“We don’t know what happened to him, and this is the bottom line,” Ms Sammarco said in a press conference to respond to the parents’ claims.

“For someone who has been bedridden for more than a year, his body was in excellent condition,” she added.

Responding to the claims about Mr Warmbier’s teeth, Ms Sammarco said she was “surprised” at the parents’ statement. "I felt very comfortable that there wasn't any evidence of trauma,” she said.

While “a few small scars” could be explained by medical procedures, she admitted there was one that remained unexplained.

The coroner’s statement also acted as a rebuttal to Donald Trump, who took to Twitter to praise the parents’ “great interview” on Fox and Friends. “Otto was tortured beyond belief,” he added.

Speaking of the moment they were renunited with their son, Cindy Warmbier said: "We thought he was in a coma, but you couldn't call it a coma. What we pictured, because we're optimists, was that Otto would be asleep and maybe in a medically-induced coma. And then when our doctors here would work with him and he'd get the best care and love, that he would come out of it."

Her husband continued: "We were in a waiting room with the medical team as the plane arrived. They went on the plane, five minutes or so later they came down and they said it was time for our family to go to the plane.

"When we got halfway up the steps, we heard this howling, involuntary, inhuman sound. We weren't really certain what it was. We climbed to the top of the steps and we looked in, and Otto was on the stretcher ... He was jerking violently, making these inhuman sounds.

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"Otto had a shaved head, he had a feeding tube coming out of his nose, he was staring blankly into space. He was blind, he was deaf. As we looked at him and tried to comfort him it looked like someone had taken a pair of pliers and rearranged his bottom teeth."

Branding the North Korean regime as “terrorists”, Mr Warmbier said: "They kidnapped Otto, they tortured him, they intentionally injured him. They are not victims, they're terrorists."

Mr Warmbier was convicted during a show trial after allegedly attempting to steal a propaganda poster from his hotel.