They described 22-year-old Otto Warmbier as in a state of "unresponsiveness wakefulness".
(CNN) - After 17 months of detention in North Korea, the conditions of which are still unclear, Otto Warmbier returned to his home state of Ohio Tuesday night.
Foreign nationals have also been detained on political grounds, including two USA university professors in Pyongyang who were arrested this year for allegedly plotting anti-state acts.
Warmbier experienced extensive tissue loss in all regions of the brain, doctors said, but he showed no signs of physical trauma and medical tests offered no conclusive evidence as to the cause of his neurological injuries.
Fred Warmbier said the family did not believe North Korea's story that his son had fallen into a coma after contracting botulism and being given a sleeping pill.
A United Nations Special Rapporteur has called for respect for the human rights of detainees in North Korea, after the release of USA student Otto Warmbier earlier this week.
U.S. doctors said they found no evidence of active botulism, a rare, serious illness caused by contaminated food or a dirty wound. Fred Warmbier said the country kept Otto's condition secret and "denied him top-notch care".
Family says Otto Warmbier has been in a coma since March 2016, shortly after being sentenced to 15 years of hard labour. 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.
"The North Koreans lure Americans to travel to North Korea via tour groups run out of China, who advertise slick ads on the Internet claiming no American ever gets detained off of our tours, and this is a safe place to go".
Fred Warmbier called on North Korea to release other American detainees.
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He praised Otto's "adventurous side" and said it was "tough to process" how his son was treated by the North Korean regime.
"But at least the ones who love him so much can now take care of him and be with him", he said, urging Americans to keep the family "in our hearts and prayers".
The next time the world saw Warmbier he was distraught, breaking down in front of Korean journalists in a video released by North Korea in February of 2016.
Kanter said while Otto has spontaneous moments of eye opening, he has not spoken or made any purposeful movements. He had not been seen in public since then. At the time the US said it was working to secure Warmbier's release.
North Koreans charged it was at that point Warmbier allegedly went into a staff-only area of the hotel and tore down a propaganda banner hanging on the wall, intending to steal it. He spoke without wife Cindy at his side because he said she stayed at the hospital with their son.
Retired basketball star Dennis Rodman called his latest trip to North Korea "really good" upon his return on Saturday.
Bill Richardson, a former USA ambassador to the United Nations, called for an investigation into what happened to Warmbier leading to this "tragic situation".
North Korea released Warmbier six days later.
Fred Warmbier, in an emotional news conference in OH, did not demur when asked if Obama could have done more.
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