Doctors say Mr Warmbier had no trace of botulism in his system, but has a severe brain injury and was in a state of "unresponsive wakefulness".
Warmbier was unable to communicate when he returned home, having suffered serious brain loss during his time behind bars.
The economics student from the University of Virginia was travelling in North Korea as a tourist when he was arrested on 2 January 2016. He was convicted and sentenced to 15 years of hard labor.
Last week North Korea repatriated Warmbier "on humanitarian grounds" to his hometown of OH after a visit by US State Department special envoy on North Korea Joseph Yun, who demanded the student by released.
He added: "It's a brutal regime, and we'll be able to handle it". While officials have reacted to past provocations with alarm and warn that the USA will consider "all options" in response to the North's defiance, they've focused on nonmilitary actions so far, such as enlisting China's help to tighten the screws on the country. Later, shortly after his return to OH, "the countenance of his face changed - he was at peace", the statement added.
"Otto has left North Korea".
While thanking the medical staff for their efforts, they said Mr. Warmbier's death was inevitable, given the "awful, torturous mistreatment" he suffered in North Korea. The North Korean explanation for Warmbier's state was that he contracted botulism, a potentially fatal bacterial illness, and fell into a coma after taking a sleeping pill.
His parents said in a statement on the day of his release that they wanted "the world to know how we and our son have been brutalised and terrorised by the pariah regime" and expressed relief he had been returned to "finally be with people who love him".
"He was kind, generous and accomplished", Portman said.
The department also demanded the release of three other Americans who had been "illegally detained". "He was home and we believe he could sense that", his parents said.
"Sadly, he is in a coma and we have been told he has been in that condition since March 2016".