The parents of Otto Warmbier claimed the 22-year-old was tortured during his captivity in North Korea.
Hamilton County Coroner Dr Lakshmi Sammarco said: “We don’t know what happened to him and that is the bottom line.”
The student died from a lack of blood and oxygen to his brain according to the Ohio coroner, who admitted the exact cause of death was a mystery.
North Korea claimed botulism and a sleeping pill was to blame as they dismissed accusations of torture.
Dr Sammarco found that the June 19 death was due an unknown injury that happened more than a year prior.
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His father, Fred Warmbier, told Fox News: “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.”
The coroner claimed there was no evidence of damage to the teeth or broken bones but did confirm multiple scars including a 4.3 by 1.6 inch wound on his right foot.
President Donald Trump tweeted that Mr Warmbier had been “tortured beyond belief by North Korea” after watching the interview with his parents on Fox.
Mr Warmbier was arrested as he was waiting to leave the hermit kingdom that he visited as part of a tour group.
The University of Virginia student was later jailed after a short show trial for attempting to steal a propaganda poster from his hotel.
He was sentenced to 15 years of hard labour but released after just 17 months and returned to the US in a coma.
The coroner carried out an external examination but no full body autopsy in keeping with the parent’s wishes.
During a rare TV appearance, the 22-year-old parents claimed Kim Jong-un’s regime had “destroyed” their son.
Branding North Korea “terrorists”, they claimed their son was deaf, blind, jerking uncontrollably and making “inhuman” sounds.
Mr Warmbier said: “When we got halfway up the steps of the plane we heard this howling, involuntary inhuman sound we weren’t really certain what it was.
“We climbed to the top of the steps we looked in Otto was on the stretcher jerking violently making these inhuman sounds.”