The 22-year-old had spent more than a year in North Korean detention after being arrested for stealing a political poster from a hotel.
Warmbier's family said they were told by North Korean officials, through contacts with American envoys, that Warmbier fell ill from botulism some time after his trial and lapsed into a coma after taking a sleeping pill, the Washington Post reported.
Three other American citizens - Kim Hak-song, Kim Sang-duk and Kim Dong Chul - remain detained by the reclusive dictatorship.
"Otto has been terrorized and brutalized for 18 months by a pariah regime in North Korea", the father said in an interview with Tucker Carlson of Fox News, scheduled to air Thursday night.
Speaking from Cincinnati Medical Center Thursday afternoon doctors treating Otto Warmbier gave an update on his medical condition.
Warmbier's father, Frank Warmbier, addressed the media earlier in the day, saying it was "bittersweet" to have his son home. Warmbier had visited North Korea as part of a tour organized by a China-based company. After making a tearful confession before the cameras, he was sentenced to 15 years' hard labor.
They kept quiet "on the false premise that (North Korea) would treat Otto fairly and let him go", he said.
SKorean president calls for NKorean Olympic participation
Pyongyang has so far rejected offers of humanitarian aid and request for a reunion of families separated by the division of Korea. Its development has been complicated by the bitter rivalry between the communist North and capitalist South.
Led coalition downs Iran-made drone in Syria
The threat was seen as retaliation for the USA downing of a Syrian air force jet on Sunday, as tensions in the region rose.
Al-Qaeda hostage Johan Gustafsson freed after six years in captivity
Sweden's Foreign Ministry said al-Qaida has released Johan Gustafsson , who was held hostage in Mali since 2011. Gustafsson was on a motorcycle trip from Sweden to South Africa when he was kidnapped .
Tests did not turn up evidence of botulism, which the North Koreans had claimed Warmbier contracted before being given a sleeping pill and not waking up, doctor Brandon Foreman said.
Fred Warmbier said the same Thursday. Joseph Yun, Washington's special envoy on North Korea, added they were working to secure their freedom as well.
During his statement, Fred Warmbier rejected North Korea's explanation for his son's condition.
"Now we know why they wouldn't allow consular access", he said.
Doctors in Cincinnati say an American college student who was imprisoned in North Korea suffered a severe neurological injury.
The retired sports celebrity was credited with the release of another USA captive in 2014, but Warmbier's release was negotiated between the two governments. He also said that he "doesn't see a tough approach to North Korea".
As for whether former President Barack Obama's administration could have done more to bring Otto home, the father said he thought "the results speak for themselves".