When he was returned to the U.S. this month on "humanitarian grounds", he had already suffered extensive brain damage and was in a vegetative state.
"Unfortunately, the bad, torturous mistreatment our son received at the hands of the North Koreans ensured that no other outcome was possible beyond the sad one we experienced today", Fred Warmbier said while announcing his son's death.
The North's spokesman said such accusations are part of a smear campaign to slander the country that had given "medical treatments and care with all sincerity" to a person who was "clearly a criminal". U.S. Sen. Rob Portman, a Republican from the Cincinnati area, told reporters that North Korea must be held accountable for Warmbier's detention and death.
He was serving a sentence of hard labor after being accused of stealing a propaganda poster.
The 22-year-old was returned comatose to the USA last week with a severe neurological injury after 18 months in captivity in the Hermit Kingdom.
Warmbier was scheduled to graduate this year from the University of Virginia's McIntire School of Commerce.
North Korea still holds Kim Sang Duk and Kim Hak-song, academics who worked at the Pyongyang University of Science and Technology, and a businessman named Kim Dong Chul.
Warmbier, 22, was arrested in the reclusive country while visiting as a tourist.
"The smear campaign against [North Korea] staged in the USA compels us to make firm determination that ... we should further sharpen the blade of law", said a spokesman for the North Korean Foreign Ministry in a statement.
Warmbier was laid to rest in Wyoming, Ohio, on Thursday, June 22.
The spokesman said it was a "mystery" as to why Mr Warmbier died days after returning home, but compared his death to the case of another American detainee, Evan Hunziker.
"Our related institutions are treating criminals who committed crimes against (our) republic strictly based on domestic law and global standards, and Warmbier was no different", the comments published by KCNA said.
His family and others have blamed North Korea for his condition.
The spokesman said that Warmbier was provided with proper medical treatment, and questioned why he died so soon after returning to the US.
He was released earlier this month in what North Korea described as a "humanitarian gesture".
Vagonis, who dated Warmbier for more than a year before he left for North Korea, recalled how the two met at a party at Warmbier's apartment, where she noticed his "insane" tie collection.
The United States has demanded North Korea release three other US citizens it holds in detention: missionary Kim Dong Chul and academics Tony Kim and Kim Hak Song.