Neurologist Daniel Kanter told a news conference in Warmbier's home city of Cincinnati, Ohio that his neurological condition was best described "as a state of unresponsive wakefulness."
"He has spontaneous eye opening and blinking. However, he shows no signs of understanding language, responding to verbal commands or awareness of his surroundings," he said.
An MRI scan revealed "extensive loss of brain tissue in all regions of the brain," Kanter said.
Warmbier's medical team said they had found no evidence of botulism, the explanation given by the North Korean regime for how the young man fell into a coma shortly after being sentenced in March 2016 for stealing a political poster from a hotel.
"We have no certain or verifiable knowledge of the cause or circumstances of his neurological injury," Kanter said.
The doctors said Warmbier's severe brain injury was most likely -- given his young age -- to have been caused by cardiopulmonary arrest cutting the blood supply to the brain.
"Based upon our analysis of those images, the brain injury likely occurred in the preceding weeks," he said.
Kanter added that body scans had revealed no evidence of acute or healing fractures to the chest, abdomen or skull.
Warmbier's father Fred lashed out earlier Thursday at the North Korean regime, saying he did not believe its claim that his son contracted botulism and fell into a coma shortly after being arrested last year.
The university student, who had been on a tourist trip, was sentenced to 15 years hard labor, a punishment the US decried as far out of proportion to his alleged crime, accusing the North of using him as a political pawn.
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