Doctors said they don't know what caused the brain damage. When asked whether it could be the result of beating or other violence while in prison, they said that Warmbier did not show any obvious indications of trauma, nor evidence of either acute or healing fractures.
Rather, Daniel Kanter, medical director of the neuroscience intensive care unit at University of Cincinnati Medical Center said, the pattern of brain injury they see on magnetic resonance imaging (MRI) results appeared consistent with a cardiopulmonary arrest, with damage to brain tissue caused by lack of blood flow to the brain.
The doctors said they are not aware of anything from his previous medical history, prior to his time in North Korea, that might cause cardiopulmonary arrest. One of the more common causes of cardiopulmonary arrest is respiratory arrest, said Jordan Bonomo, neurointensivistand emergency medicine physician at UC Health. That cessation of breathing could be triggered by several things, including intoxication or a traumatic injury.
It is possible to have respiratory arrest caused by an overdose of medication, intentional or otherwise, he said.
Otto Warmbier's condition, its possible causes and his treatment while he was detained in North Korea are of intense interest in a case that threatens to worsen already fraught relations between the United States and North Korea. President Trump called Warmbier's parents Wednesday night to tell them his administration had worked hard to secure their son's release, and to ask how he was doing.
Secretary of State Rex Tillerson also expressed relief that Warmbier has been returned to the United States. However, asked at news conference in Miami about how the United States might respond given Warmbier's brain damage during captivity, Tillerson said he would not comment on his health, saying it was a private matter for the Warmbiers. "We are glad that he is home, rejoined with his family," he said.
Warmbier has undergone a battery of tests since his arrival at the UC Medical Center on Tuesday night. Throughout that time, his family has been at his bedside, doctors said.
They declined to discuss his prognosis or speculate about the future, at the family's request.
The doctors said he has spontaneous eye-opening and blinking but shows no signs of understanding language or awareness of his surroundings. He has not spoken or engaged in any purposeful movement, Kanter said.
"We don't feel he has any conscious awareness," Brandon Foreman, neurointensive care specialist at UC Gardner Neuroscience Institute, said.
They have had no direct contact with North Korean medical authorities, Kanter said, but Warmbier arrived with two brain scans dated April and July 2016. They don't have any way to verify those dates, but the damage to the brain is consistent with the deterioration they see from those previous scans, he said. After the tissue is damaged initially by insufficient blood flow - which they think probably happened before that initial scan - the body tries to remove the damaged tissue. "That's what we're seeing is this removal."
It has been almost a year and a half since Warmbier was detained in North Korea, which he had visited on a five-day tourist trip on his way to a study-abroad trip to Hong Kong with the University of Virginia. On his last night there, he apparently tried to remove a large propaganda sign. He was charged with "hostile acts against the state" and, after a sham trial, sentenced to 15 years of hard labor.
It has been about 15 months since all contact was severed. He was allowed no consular visits.
They were told - but did not believe - that shortly after the trial Otto Warmbier contracted botulism and was given a sleeping pill, and that he had never recovered.
There was no evidence indicating botulism, Foreman said Thursday.
Joseph Yun, the State Department's special representative for North Korea, rushed to help secure Warmbier's medical evacuation. Late Tuesday night, the Gulfstream jet touched down in Cincinnati, and his family finally got to see him, before medical personnel carried him off the plane into a waiting ambulance.
At a news conference earlier on Thursday, Fred Warmbier said they are left with many questions and no answers.
One many people had was how an athletic young man had become so desperately ill.
"There is no excuse for any civilized nation" to have kept his condition a secret and denied him medical care for so long, Fred Warmbier said.
On Thursday, North Korea's state-run news agency said that a court had allowed Warmbier to return home "on humanitarian grounds."
When asked about that, Fred Warmbier said he thinks the State Department was pretty tough with them. "They did not do this out of the kindness of their hearts."
Otto Warmbier was a top student at the University of Virginia who was planning to go into finance, with a summer internship lined up before he left the country in December 2015, a close-knit family and a huge crowd of friends from home and college.
He had, as his mother said a month ago, a lot to lose.
On Thursday, his father said his son's spirit is with them: "He's with us. We're trying to make him comfortable. We want to be part of his life."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)