(CNN) Doctors caring for released North Korea detainee Otto Warmbier said he has not spoken or moved on his own since he arrived in the United States on Tuesday, a condition they described as "unresponsive wakefulness" or persistent vegetative state.

The 22-year-old has suffered extensive loss of brain tissue in all regions of the brain, doctors at the University of Cincinnati Medical Center said in a news conference Thursday.

Also known as persistent vegetative state, the syndrome's symptoms include no voluntary movement or awareness of surroundings. Warmbier opens his eyes and blinks spontaneously but shows no signs of understanding language or responding to verbal commands, said Dr. Daniel Kanter, professor of neurology and director of the Neurocritical Care Program.

The news shed light on the Warmbier family's statement that their son suffered severe brain damage at some point in his 17 months of detention.

His parents said they learned of their son's condition -- what North Korea called a coma -- only last week. Earlier Thursday, Fred Warmbier said he rejected the regime's explanation that his son fell into a coma after contracting botulism and taking a sleeping pill in March 2016 after his trial for trying to steal a political banner.

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The doctors said they had no information about the care he received in North Korea. Though they could not say with certainty what caused his injuries, they found no evidence to support the botulism claim. But an analysis of images from North Korea of Warmbier's brain dated April 2016 suggests the injury occurred in the preceding weeks.

"We have no certifiable knowledge of the cause or circumstances of his neurological injuries," Kanter said.

"This pattern of brain injury is usually seen as result of cardiopulmonary arrest where blood supply to (the) brain is inadequate for a period of time, resulting in the death of brain tissue."

Conviction and release

Otto Warmbier was a University of Virginia student when he was detained in January 2016 at Pyongyang airport on his way home. He had been on a tour of the reclusive country, his parents said.

North Korean authorities claimed they had security footage of him trying to steal a banner containing a political slogan that was hanging from a wall of his Pyongyang hotel.

That was used as evidence in his hourlong trial. He was found guilty of committing a "hostile act" against the country and sentenced in March 2016 to 15 years of hard labor. It was the last time he was seen publicly before this week.

"Even if you believe their explanation of botulism and a sleeping pill causing a coma -- and we don't -- there is no excuse for any civilized nation to have kept his condition a secret and denied him top-notch medical care for so long," said Fred Warmbier in a 23-minute news conference at his son's alma mater, Wyoming High School, north of Cincinnati.

The father saluted his son as a brilliant, adventurous and courageous man who did what he could to endure brutality and terror.

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The father, wearing the cream sport coat his son wore during his televised trial in North Korea, stopped short of saying how he believed his son was injured.

"We're going to leave that to the doctors today," he said.

Though they could not provide a specific cause, the doctors said Warmbier showed spastic, profound weakness and contractions in his muscles and legs. Beyond minor skin blemishes consistent with medical care they found no evidence of fractures or trauma to his body.

Nor did they find evidence of botulism, a toxin that causes nerve injury, Dr. Brandon Foreman said. Because it does not stay in the body for long doctors looked for signs of chronic denervation, which is loss of nerve supply, he said. "We did not find any evidence of that."

Critical of Obama administration

Fred Warmbier appeared critical of the Obama administration's handling of Otto's detention, saying the family heeded the US government's initial advice to take a low profile "without result."

They kept quiet "on the false premise that (North Korea) would treat Otto fairly and let him go," he said.

He said he and his wife, Cindy, decided this year that the "time for strategic patience was over," and so they did media interviews and traveled to Washington to meet the State Department's special representative for North Korean policy, Joseph Yun

Yun met in May with North Korean representatives in Norway, and the North Koreans agreed that Swedish representatives would be allowed to visit Otto Warmbier and three other US detainees, a senior State Department official said on condition of anonymity this week.

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After the Swedes visited one detainee, North Korea representatives sought another meeting with Yun, and it was at that June 6 meeting in New York that North Korea's UN ambassador told Yun that Warmbier was in a coma, the official said.

North Korea released Warmbier six days later.

Fred Warmbier praised the Trump administration's efforts: "They have our thanks for bringing Otto home."

When asked whether then-President Barack Obama could have done more, Warmbier replied, "I think the results speak for themselves."

Three other US detainees

Warmbier's release coincided with basketball star Dennis Rodman's latest visit to North Korea , though US national security spokesman Michael Anton said there is no connection between the two.

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Fred Warmbier said the same Thursday.

"Dennis Rodman had nothing to do with Otto," he said.

Rodman was asked by reporters Tuesday if he would bring up the cases of Warmbier and three other Americans detained in North Korea. "That's not my purpose right now," he said. "My purpose is to go over there and try to see if I can keep bringing sports to North Korea."

The other Americans held by Pyongyang are Kim Sang Duk and Kim Hak-song, academics who worked at the Pyongyang University of Science and Technology, and a businessman named Kim Dong Chul.

Fred Warmbier called on North Korea to release other American detainees.

"There's no excuse for the way the North Koreans treated our son. And no excuse for the way they've treated so many others," he said. "No other family should have to endure what the Warmbiers have."