The 22-year-old is now being treated in hospital after the flight carrying him landed in Ohio on Tuesday.
Mr Warmbier was sentenced to 15 years of hard labour for attempting to steal a propaganda sign from a hotel.
He was given a sleeping pill after becoming ill after his trial last year and did not wake up, North Korea said.
He is now being treated at the University of Cincinnati Medical Center.
How harsh is prison in North Korea?
"We want the world to know how we and our son have been brutalised and terrorised by the pariah regime," his parents, Fred and Cindy, said.
Former US ambassador and New Mexico governor Bill Richardson, who has previously served as special envoy to North Korea and in 2016 attempted to negotiate the release of the student, said the Warmbiers had updated him on their son's condition.
"In no uncertain terms, North Korea must explain the causes of his coma," Mr Richardson said.
The BBC's Stephen Evans in Seoul says it remains to be seen if Mr Warmbier's illness is the direct result of brutality in prison.
If it were, there might be pressure on President Trump to take action against Kim Jong-un's regime, he says.
Otto Warmbier is an economics student from the University of Virginia, originally from Cincinnati, Ohio.
He was in North Korea as a tourist with Young Pioneer Tours when he was arrested on 2 January 2016.
He appeared emotional at a news conference a month later, in which he tearfully confessed to trying to take the sign as a "trophy" for a US church, adding: "The aim of my task was to harm the motivation and work ethic of the Korean people."
Foreign detainees in North Korea have previously recanted confessions, saying they were made under pressure.
After a short trial on 16 March, Mr Warmbier was given a 15-year prison sentence for crimes against the state.
His parents told CNN in early May that they had had no contact with their son for more than a year.
In a statement on Tuesday, Mr and Mrs Warmbier said: "Sadly, he is in a coma and we have been told he has been in that condition since March 2016. We learned of this only one week ago."
They were quoted by the Washington Post as saying they had been told Otto had contracted botulism, a rare illness that causes paralysis, soon after his trial in March 2016.
He was given a sleeping pill and had been in a coma ever since, the newspaper said.
Secretary of State Rex Tillerson made no mention of Mr Warmbier's condition in a statement, saying only that he was on his way home to be reunited with his family and would not make any further comment, out of respect for the privacy of the family.
There are reported to be three other US citizens in custody in North Korea. They are:
The US has in the past accused North Korea of detaining its citizens to use them as pawns in negotiations over its nuclear weapons programme.
The arrests have come at a time of heightened tension between North Korea and the US and its regional neighbours.
Mr Warmbier's release comes hours after US basketball star Dennis Rodman arrived in North Korea.
Mr Rodman is a friend of North Korean leader Kim Jong-un and has made several visits to the country in recent years.
There was some speculation that he might plead the case for the American detainees, but en route to North Korea he told reporters "my purpose is to actually see if I can keep bringing sports to North Korea".
The parents of Otto Warmbier, the US student who is in a coma after being freed this week by North Korea, say he was "brutalised" by a "pariah regime".