Warmbier, an American student, was imprisoned in North Korea and was apparently abused and beaten.
The doctors said Warmbier's brain injuries are consistent with respiratory arrest, possibly caused by intoxication or traumatic injury.
A White House spokeswoman says Warmbier's release "was a big priority" for President Donald Trump.
"While I welcome the news of Mr Warmbier's release, I am very concerned about his condition, and the authorities have to provide a clear explanation about what made him slip into a coma", Tomas Ojea Quintana, the United Nations special rapporteur on human rights in the Democratic People's Republic of Korea (DPRK), said in a statement issued in Geneva.
Fred Warmbier praised the Trump administration's efforts: "They have our thanks for bringing Otto home".
Bae was released with another American, Matthew Todd Miller, who had served less than two months of his six-year sentence.
Spokeswoman Heather Nauert (NOW'-ert) says the US hopes the other Americans will be able to return home soon.
The North Korean government said botulism is to blame for Warmbier's condition, but doctors haven't found any evidence of the illness in the now 22-year-old. He was returned to his hometown in suburban Cincinnati in a coma. Kanter described his condition as a being in a state of "unresponsive wakefulness".
"However, he shows no signs of understanding language, responding to verbal commands or awareness of his surroundings". Tourism is very welcomed in North Korea, thus tourists are cherished and well taken care of.More news: Duterte resurfaces after rest, says Islamist insurgency has deep roots
Fred Warmbier appeared critical of the Obama administration's handling of Otto's detention, saying the family heeded the USA government's initial advice to take a low profile "without result".
White says Warmbier was a master at defusing tense situations with his humor. The photos brought home the disturbing news that the University of Virginia student was effectively comatose because of a brain injury. North Korea has a long history of kidnapping or arbitrarily detaining foreigners. Late Tuesday night, the Gulfstream jet touched down in Cincinnati, and his family finally got to see him, before medical personnel carried him off the plane into a waiting ambulance.
In a news conference on Thursday, Warmbier's father called his return bittersweet.
Warmbier was sentenced to 15 years hard labor in March last year after traveling to North Korea while he was a student at the University of Virginia. Fred Warmbier said he received "a very nice phone call" Wednesday from the president. His family was unaware of his condition until he was released.
The U.S. recently obtained intelligence reports suggesting that North Korean authorities repeatedly beat Warmbier while he was in custody, a senior American official told The New York Times.
Dr. Jordan Bonomo said Warmbier, 22, has "no fractures to the bone and has minor blemishes on his skin. But what they do is they provide fodder for the North Koreans and my son happened to become fodder for the North Koreans", Fred Warmbier said.
An American university student who was returned to the U.S. this week after being held in North Korea for 18 months, has a severe brain injury and is in a state of "unresponsive wakefulness", doctors say.
Warmbier's parents were told their son was given a sleeping pill soon after his trial in March a year ago but never woke up.
"Earlier this year, Cindy and I decided the time for strategic patience was over, and we made a few media appearances and traveled to Washington to meet with Ambassador Joe Yun at the State Department", he continued.