Mr Warmbier said his son was "terrorised and brutalised" during his 17-month detention, and has been in a coma for more than a year.
Warmbier's release coincided with basketball star Dennis Rodman's latest visit to North Korea, though Michael Anton, a United States national security spokesman, told CNN there is no connection between the two. Richardson credited the State Department with the successful return of Warmbier, but said a response by the USA government will be required if it is found there was a cover-up relating to Otto's condition and if he had not received proper medical treatment. He does not believe North Korea's claim that his son's coma resulted from botulism and a sleeping pill. "I don't know what being in shock is, but I'm pretty sure I was", he said, referring to being informed Otto Warmbier was in a coma.
After a Pyongyang diplomat told ABC News last summer that USA citizens detained in North Korea would be subjected to wartime laws, Richardson suggested that the Geneva Conventions might be at stake. But they provide fodder for the North Koreans. "No other family should have to endure what the Warmbiers have", he added.
While at the Pyonyang hotel, where Warmbier was staying, the then 21-year-old was allegedly seen by security cameras, taking a political banner from the hotel. They did not find evidence of broken bones or other physical trauma, and they said Warmbier's brain injury was not consistent with a beating.
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The North Korean regime deemed him a "war criminal" and "brutalized and terrorized" him during his detainment, his father said. Asked if the administration of former President Barack Obama had done enough to secure their son's release, Warmbier said: "The results speak for themselves".
Otto Warmbier is taken to North Korea's top court in Pyongyang in March 2016.
The 22-year-old University of Virginia student returned to his OH home in a coma earlier this week after spending 17 months in the North on charges of stealing a political propaganda poster. He was convicted of "hostile acts" against the country and sentenced to 15 years hard labor.
Fred Warmbier said he was stunned when told of his son's condition a week ago.
The family was "extremely grateful for their efforts and concern", Fred Warmbier said.
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