Mr Warmbier, 22, is due to arrive home in Cincinnati on Tuesday evening, after a stop at a US military facility in Sapporo, Japan.
Mr Warmbier's parents, Fred and Cindy, said in a statement that they have been told their son has been in a coma since March 2016, and they had learned of this only one week ago.
They said: “We want the world to know how we and our son have been brutalised and terrorised by the pariah regime” in North Korea.
They also said they are grateful he “will finally be with people who love him.”
Mr Warmbier, a student at the University of Virginia, went with a tour group from Beijing to Pyongyang in January 2016 despite US State Department travel warnings.
He was arrested and convicted while trying to leave the country after his group's “New Year’s Party Tour”.
The charge, according to North Korean news agency, was “perpetrating a hostile act against” the Democratic People's Republic of Korea – the official name for the reclusive Communist nation.
Mr Warmbier allegedly attempted to steal a propaganda banner from the hotel in which he was staying, but it remains unclear whether this is true.
Approximately six weeks after his arrest – in March 2016 – the Ohio native was sentenced to 15 years hard labour.
Mr Warmbier fell ill from botulism sometime after his March trial and fell into a coma after taking a sleeping pill.
In previous situations when the US government has wanted an American citizen released, at least one senior official has had to travel to Pyongyang in order to secure their release.
Bill Richardson, a former Democratic congressman, UN Ambassador, US energy secretary and ex-Governor of New Mexico, welcomed Mr Warmbier's release but said “we are deeply concerned regarding his health”.
Senator Rob Portman of Ohio said: “Otto’s detainment and sentence was unnecessary and appalling, and North Korea should be universally condemned for its abhorrent behaviour. Otto should have been released from the start.”
Former President Bill Clinton went in 2009 in order to get two journalists, Euna Lee and Laura Ling, released and former CIA director James Clapper visited in November 2014 to bring home Korean-American missionary Kenneth Bae and Matthew Miller, who was charged with espionage.
It does not appear than any senior officials from the Trump administration have made a trip to North Korea. But Mr Warmbier's release came as former US basketball star Dennis Rodman arrived in North Korea on Tuesday, returning to the increasingly isolated nuclear-armed country where he has previously met leader Kim Jong Un.
A senior administration official said the Trump administration did not authorise Rodman's trip. “This is him freelancing,” the official told Reuters.
In late April however, he told CBS that: “At a very young age, [Mr Kim] was able to assume power. A lot of people, I'm sure, tried to take that power away, whether it was his uncle or anybody else. And he was able to do it. So obviously, he’s a pretty smart cookie.”
Defence Secretary James Mattis declared North Korea “most urgent and dangerous threat to peace and security,” before the House Armed Services Committee, just hours before Mr Warmbier's release was announced.
“North Korea’s continued pursuit of nuclear weapons and the means to deliver them has increased in pace and scope,” Mr Mattis said.
And Mr Tillerson – speaking in front of the Senate Foreign Relations Committee → said that Washington is discussing North Korea with all of its allies, and seeing some response from China. He said North Korea would be at the top of the agenda next week at a high-level meeting between US and Chinese officials.
Mr Tillerson said the United States would have to work with other countries to deny North Korea access to basics such as oil and will have to consider whether to impose secondary sanctions on those doing business with North Korea.
“We are in a stage where we are moving into this next effort of, ‘Are we going to have to, in effect, start taking secondary sanctions because countries we have provided information to have not, or are unwilling, or don't have the ability to do that?’” Mr Tillerson said.
At the same hearing, the Secretary of State said that the US relationship with Russia is at an all-time low and deteriorating further, yet he cautioned against taking steps that might close off promising avenues of communication between the two former Cold War foes.
Mr Tillerson stopped short of registering his opposition to a new package of Russia sanctions the GOP-led Senate is considering in retaliation for Moscow's meddling in the 2016 presidential election and its aggression in other parts of the world, including Syria and Ukraine.
Mr Tillerson told the committee that he's still reviewing the new sanctions that Senate Republicans and Democrats agreed upon late Monday after lengthy negotiations. He said that it's important that Mr Trump have the flexibility “to turn the heat up” on Russia if necessary. He also said he doesn't want promising channels of communication preemptively shut down.
Talks with Moscow on stabilising war-ravaged Syria are progressing but it's too early to tell if the discussions will bear fruit, Mr Tillerson said.
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies