WYOMING, an American college student who emerged from prison in North Korea in a coma has severe brain damage, but doctors do not know what caused it, a medical team treating him in Ohio said Thursday.

The doctors described Otto Warmbier, 22, as being in a state of "unresponsive wakefulness," but declined to discuss his outlook for improvement, saying such information would be kept confidential.

"He has spontaneous eye opening and blinking," said Dr. Daniel Kanter, director of neurocritical care for the University of Cincinnati Health system. "However, he shows no signs of understanding language, responding to verbal commands or awareness of his surroundings. He has not spoken."

Warmbier is in stable condition at the UC Medical Center, where he was taken immediately after his arrival in Ohio late Tuesday after more than 17 months in North Korean captivity. The reclusive country accused the University of Virginia student of anti-state activities.

His father, Fred Warmbier, met with reporters earlier and said the family was proud of Otto, calling him "a fighter." He said the family is trying to keep him comfortable.

The elder Warmbier said he didn't believe North Korea's explanation that the coma resulted from botulism and a sleeping pill. U.S. doctors said they found no evidence of active botulism, a rare, serious illness caused by contaminated food or a dirty wound.

He said there was no reason for North Korea to keep his son's condition apparently dating to March 2016 a secret and to deny him top medical care.

Kanter said the youth suffered "extensive loss of brain tissue in all regions of the brain." Doctors said his injuries are consistent with respiratory arrest cutting off oxygen to the brain, but they're not certain what caused it.

Doctors said they also examined the rest of Warmbier's body and saw no evidence of fractures in his skull or elsewhere that might be evidence of severe beatings.

The family feels "relief that Otto is now home in the arms of those who love him and anger that he was so brutally treated for so long," his father said at Wyoming High School, where Warmbier graduated in 2013 as class salutatorian and played soccer. Blue-and-white ribbons in the school's colors were tied around trees and utility poles all the way along the city's main road in a show of support.

To honor his son, Fred Warmbier wore the same jacket Otto wore when North Korea presented him before the media on Feb. 29, 2016, at an event where he tearfully confessed that he tried to steal a propaganda banner while visiting the country. He was last seen publicly that March, when he was sentenced for subversion to 15 years in prison with hard labor.

Fred Warmbier said that he doesn't know why North Korea released his son but that the country doesn't do anything out of "the kindness of their hearts." He called on the country to release three other Americans currently held there."There's no excuse for the way the North Koreans treated our son," he said.

In its first official comment since Warmbier was returned home, North Korea said it released him for humanitarian reasons. The state-run Korean Central News Agency on Thursday did not comment on his medical condition.

Warmbier's father also accused North Korea of luring Americans to the country with a Chinese tour company making the false promise they will never be detained.

He said he received "a very nice phone call" Wednesday evening from President Donald Trump, who said Secretary of State Rex Tillerson worked hard to bring Otto home. Warmbier said the family was "extremely grateful for their efforts and concern."

He said he and his wife grew frustrated with the lack of word about their son from former President Barack Obama's administration, which they said instructed them to keep a low profile to avoid upsetting the North Koreans.

Asked whether he thought the previous administration could have done more, Fred Warmbier replied: "I think the results speak for themselves."

Bill Richardson, a former U.S. ambassador to the United Nations, called for an investigation into what happened to Warmbier.

Richardson, a Democrat, credited the State Department with securing Warmbier's return from North Korea without any preconditions but said a forceful response from the U.S. government would be required "if it's determined that there was a cover-up and Otto's condition was not disclosed and he didn't get proper treatment."

Such detentions in the totalitarian nation have added to tensions between Washington and Pyongyang.

The U.S. government accuses North Korea of using such detainees as political pawns. North Korea accuses Washington and South Korea of sending spies to overthrow its government.

Tillerson said Tuesday that the State Department was continuing "to have discussions" with North Korea about the release of the other imprisoned Americans.

-AP