WYOMING, Ohio — 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 in the day 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 did not believe North Korea’s explanation that the coma resulted from botulism and a sleeping pill.