WYOMING, Ohio -- An American college student who was imprisoned in North Korea and returned to his home state of Ohio in a coma suffered a "severe neurological injury," a hospital spokeswoman said Thursday.

Otto Warmbier is in stable condition at the University of Cincinnati Medical Center with his mother by his side, hospital spokeswoman Kelly Martin said. Doctors planned an update later Thursday.

His father, Fred Warmbier, said he does not believe North Korea's explanation that the coma resulted from botulism and a sleeping pill. He said there was no reason for North Korea to keep his son's condition a secret and deny him top medical care.

American student Otto Warmbier is presented to reporters on Feb. 29, 2016, in Pyongyang, North Korea, following his arrest. AP

Fred Warmbier revealed during the press conference that he received a phone call from President Trump on Wednesday night inquiring about his son's condition. He described the conversation as kind and gracious.

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Warmbier said that his son's return has been bittersweet.

"Relief that Otto is now home in arms of those who love him and anger that he was so brutally treated for so long," he said at a news conference at Wyoming High School, where Warmbier graduated in 2013 as class salutatorian and played soccer.

He told Fox News' Tucker Carlson on Wednesday that Otto was "terrorized and brutalized" during his 17-month detention and has been in a coma for more than a year.

"The day after he was sentenced, he went into a coma," the father said in an interview scheduled to air Thursday night. He said he and his wife, Cindy, only learned of their son's condition last week.

The 22-year-old University of Virginia student was medically evacuated from North Korea and arrived in Cincinnati late Tuesday. He was then taken by ambulance to the University of Cincinnati Medical Center.

Residents of the northern Cincinnati suburb tied blue-and-white ribbons, the school colors, to trees near the family's home. Joy at his release was mixed with concern over his condition.

Alison Lebrun, center background, helps tie blue-and-white awareness ribbons along Springfield Pike near the family home of Otto Warmbier on Tuesday, June 13, 2017. John Minchillo / AP

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 said he had been sentenced to hard labor but did not comment on his medical condition.

Former New Mexico Gov. Bill Richardson, a former U.S. ambassador to the United Nations who has negotiated with the North Korean regime in the past, said there should be an investigation into what happened to Warmbier leading to this "tragic situation."

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."

Warmbier was serving a 15-year prison term with hard labor in North Korea after he tearfully confessed that he tried to steal a propaganda banner while visiting the country.

Such detentions in the totalitarian nation have added to tensions between Washington and Pyongyang. Three Americans remain in custody.

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.

Secretary of State Rex Tillerson said Tuesday that his department was continuing "to have discussions" with North Korea about the release of the other three imprisoned American citizens.

When asked by Fox News what he would tell the families of those detained, Fred Warmbier said, "I wouldn't know what to say to them. This is, I've been told, not precedented."