Mr Warmbier, 22, was flown back to the US in a coma on Tuesday after 17 months in custody.
North Korea said he had fallen ill shortly after beginning a sentence of 15 years hard labour for attempting to steal a propaganda sign from a hotel.
He was given a sleeping pill after becoming ill after his trial last year and did not wake up, North Korea said.
Joseph Yun, the US state department’s special envoy to North Korea, travelled to Pyongyang with a medical team to secure his release.
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