A team of US doctors have found "no sign of botulism" in the American student freed by North Korea after more than 15 months in captivity.
The regime had said 22-year-old Otto Warmbier's coma was caused last year by botulism and a sleeping pill he took after his trial.
He has not spoken since his return to his family hometown in Ohio.
"His neurological condition can be best described as a state of unresponsive wakefulness," said Dr Daniel Kanter.
Mr Warmbier "shows no understanding of language" and has "extensive loss of brain tissue", which was likely caused by cardiopulmonary arrest, he said.
According to scans taken after he arrived at the Cincinnati Medical Center earlier this week, there was no sign that he was physically abused during his detention, his doctors said.
Mr Warmbier's condition was "not what we normally see with traumatic brain injury. It's the type we see with cardiopulmonary arrest," Dr Kanter told reporters.
The doctors believed respiratory arrest led to his condition, which was caused by a lack of oxygen and blood in the brain.
Earlier on Thursday, Mr Warmbier's father expressed doubts about the North Korean account of what caused the University of Virginia student's coma.
"Even if you believe their explanation of botulism and a sleeping pill causing the coma - and we don't - there's no excuse for a civilised nation to have kept his condition secret and to have denied him top notch medical care."
Mr Warmbier, an economics student from the University of Virginia, was arrested in January 2016 while visiting North Korea as a tourist.
He was given a 15-year prison sentence for attempting to steal a propaganda sign from a hotel.