Warmbier, who was in a coma when he was released, suffered "extensive loss of brain tissue in all regions of the brain", one of his doctors said.
Doctors declined to speculate on what may have caused Warmbier's condition and said they would not discuss his prospects or treatment out of respect for his family.
It has been nearly 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.
"He (Warmbier) has spontaneous eye opening and blinking", said Daniel Kanter, director of neurocritical care at the University of Cincinnati Health System, according to AP. His doctors say he is barely functioning and shows no signs of situational awareness.
Richardson, who has been in close contact with the Warmbier family, said he did not believe Pyongyang's explanation that the student's condition was the result of botulism and a sleeping pill. But US doctors said they found no evidence of the illness.
"There is no excuse for any civilized nation to have kept his condition secret and denied him top-notch medical care for so long", Fred Warmbier said.
The Warmbiers were told just a week ago that their son contracted botulism and fell into a coma shortly after being arrested and sentenced, in March past year. The brain can suffer permanent damage if it doesn't receive blood and oxygen for a prolonged period of time, although Warmbier's doctors did not disclose if his brain injury was irreversible.
Warmbier's parents, Fred and Cindy, confirmed their son was on a medevac flight. "North Korea has proven they are not nature's noble men".
Meanwhile, the background process of how Warmbier got to be released brings attention to the possibility of dialogue between Washington and Pyongyang on the North Korean nuclear issue.
Tillerson said on Wednesday that United States officials were considering some type of travel visa restriction to North Korea.
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Hundreds of thousands of people, including children, are detained in the inhumane political prisons and other detention facilities in North Korea and forced to work in its mines.
Trump said on Friday that what had happened to Warmbier was "a truly awful thing".
In May, Yun met in the Norwegian capital of Oslo with senior officials from North Korea's Foreign Ministry. A Young Pioneer Tours official told a news site that focuses on the region that only one person - Otto Warmbier - has ever been arrested among the 8,000-plus worldwide travelers who have taken part in the company's tours.
Trump called the family last night, Fred Warmbier said.
Otto Warmbier was a 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.
"I call on the DPRK authorities to protect all prisoners, be they North Korean or foreigners. I just said kind of quite nervously, 'Well, that's the last we'll see of you.' There's a great irony in those words".
In its first official comment since Warmbier was returned home, North Korea said it released him for humanitarian reasons. Warmbier said the family was "extremely grateful for their efforts and concern".
Mr Warmbier said the tour group "provides fodder" for the regime.
Kelly Martin of the University of Cincinnati Medical Center said Thursday that Otto Warmbier (WORM'-bir) is in stable condition after arriving at the hospital two days ago.
When asked whether then-President Barack Obama could have done more, Warmbier replied, "I think the results speak for themselves".