Fred Warmbier also credited the Trump administration for its "tough" approach to securing Otto's release, whereas he says that Obama's White House simply encouraged the parents to lay low while no progress was made.
Fred Warmbier told reporters at a press conference in Cincinnati, Ohio, on Thursday morning the release came after he and his wife decided the "time for strategic patience was over", the BBC reported.
"Relief that Otto is now home in the arms of those who love him, and anger that he was so brutally treated for so long".
Rodman, one of the only Westerners to have personally met Kim Jong Un, has been criticized for a prior trip where he sang "Happy Birthday" to Kim and suggested an American missionary was at fault for his own imprisonment in North Korea, remarks for which he later apologized.
Some time in March, Warmbier fell into a coma - but was kept in North Korea for another 14 months.
In recent months, Warmbier said, he and his wife, Cindy, had chose to take a more public stance in calling for their son's release, after months of following advice that they should try not to antagonize or offend leaders in North Korea. A source close to the family said Warmbier contracted botulism previous year.
Ohio Senator Rob Portman described the North Korean government's handling of Warmbier as "abhorrent" in an interview with Fox News.
Although he was released hours after USA basketball player DennisRodman (friend of the North Korean leader Kim Jong-un) landed in North Korea, nothing is quite clear if his arrival is linked or not.
The state-run Korean Central News Agency said Otto Warmbier had been serving hard labor but didn't comment on his medical condition or how the country negotiated his release with the United States.
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The NSA said in statement that it would "fully cooperate with the special counsel" and declined to comment further. The investigation has been cloaked in secrecy and it's unclear how many others have been questioned by the FBI .
Doctors are expected to offer further details of Otto's medical condition during a news conference later Thursday afternoon.
Fred Warmbier called his son's return bittersweet.
Voice breaking and tears in his eyes, the father noted he was wearing the same off-white jacket his son had worn during his North Korean court appearance, adding he was "proud" of his son's adventurous spirit.
President Trump called him Wednesday and asked how the family was doing but said he was sorry about Otto's condition. "We are extremely grateful for their efforts and concern", Warmbier said.
The New York Times previously cited a senior USA official as saying Washington had received intelligence reports that Warmbier had been repeatedly beaten while in North Korean custody.
Otto's father answered, "I think the results speak for themselves".
Although U.S. citizens are not banned from visiting North Korea, the U.S. State Department strongly advises against it.
Former New Mexico Gov. Bill Richardson, a former us ambassador to the United Nations and a Democrat, credited the State Department with securing Warmbier's return.
The Democrat and former U.S. ambassador to the United Nations praised the Department of State for working to bring Mr Warmbier home.
Warmbier left North Korea just a few hours after Dennis Rodman, the eccentric former basketball star, arrived, but the State Department says the events were not related and Rodman had denied on the way in that he was on a mission to free the four Americans being detained by Pyongyang.