(CNN) Days after US student Otto Warmbier died, North Korean state media said his death was a "mystery" and dismissed accusations he was tortured during his 17 months in captivity in the country.

Doctors who examined Warmbier on his return to the United States last week said the University of Virginia student had incurred significant brain damage and was in a state of " unresponsive wakefulness ." But it's unclear how he came to be in that state.

The North Koreans had told US officials that Warmbier slipped in a coma after contracting botulism and taking a sleeping pill.

The 22-year-old's family alleges he was tortured.

"Unfortunately, the awful, 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," Warmbier's parents said in announcing their son's death Monday.

But the state-run Korean Central News Agency said Friday said that Warmbier was treated in accordance with domestic law. It rebuffed "groundless public opinion now circulating in the US that he died of torture and beating during his reform through labor."

"The fact that Warmbier died suddenly in less than a week just after his return to the US in his normal state of health indicators is a mystery to us as well," the agency reported.

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Sentenced to hard labor

Warmbier was detained at the Pyongyang airport in January 2016 after a short tour of North Korea.

Two months later, he was found guilty of committing "hostile acts" against the North Korean regime and sentenced to 15 years' hard labor in a trial that reportedly lasted about an hour.

Some of the evidence at trial was purported security footage of Warmbier removing a political banner from his hotel.

More than a year went by without public word of Warmbier before the State Department learned of his dire condition on June 6. He was released a week later.

President Donald Trump called the situation a "disgrace."

"He should have been brought home a long time ago," Trump said.

Warmbier died days after his return to the United States, and his funeral was held Thursday in his hometown of Wyoming, Ohio.

Kenneth Bae, an American who was jailed for nearly two years in North Korea, told CNN he believed Warmbier could have been tortured, and cautioned other Americans against going to North Korea.

Dennis Rodman's visit

though US national security spokesman Michael Anton denied there was a connection between the two. Warmbier's release coincided with former basketball star Dennis Rodman's latest visit to North Korea though US national security spokesman Michael Anton denied there was a connection between the two.

Fred Warmbier, the student's father, said the same thing Thursday: "Dennis Rodman had nothing to do with Otto."

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But Rodman's agent told ABC News said he asked for Warmbier's release on behalf of the former athlete three times before the trip. The agent, Chris Volo, said he believes Rodman's trips to North Korea "had a lot to do" with Warmbier's release.

"I know being there had something to do with it," Volo said.

"When I was organizing the trip and I (met) with the delegates here, I addressed Otto Warmbier and I said to them we would need some type of good faith if we're ever going to do some type of future sports relations."

Volo said they hope to meet with the family but have been told "it just couldn't happen."

Rodman told ABC News he didn't know Warmbier was in a coma when he heard the news of his release.

"I was just so happy to see the kid released. Later that day, that's when we found out he was ill. No one knew that. I was just so happy. ... We jumped up and down. All right man, all right. Some good things came out of this trip."

Fred Warmbier has called on North Korea to release other American detainees.

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.

Will Rodman and Volo lobby for those Americans' release? "If we could get more trips going over there, I think anything is possible," Volo said.