Transcript for What caused Otto Warmbier's brain condition: Part 5

Text2 Test message Reporter: Shortly after those heartwrenching images of Otto being carried off the plane -- His condition can be best described as a state of unresponsive wakefulness. Reporter: The bad news gets worse. The worsening horror for a mother and father from Ohio. Their son, who was just returned from North Korea with severe brain damage, has now died. Reporter: It was not the high school homecoming that Otto Warmbier's parents could ever have imagined. His funeral was held in the auditorium of his ALMA mater, Cincinnati's Wyoming high school, yesterday. On display, the jacket he wore on the north Korean court, and the passport that ultimately did not save his life. We feel like we lost a son. He represents everything that's the best in our community and we're just offering support for the family. Reporter: After 18 months of helplessly following Otto's captivity, reaction to his shocking end washed across social media. On everyone's mind, what happened to him in North Korea? At a press conference at the hospital in Cincinnati, where he was briefly treated before his death, some clues were revealed. Our purpose today is to describe the medical and neurological condition of Mr. Otto Warmbier. Reporter: It turns thut mri images supplied by the north Koreans showed that Otto's injury was over a year old. The earliest images are dated April 2016. Based upon our analysis of those images, the brain injury likely occurred in the preceding weeks. Reporter: That's just weeks after these images were shot of Warmbier dragged from a north Korean court. This pattern of brain injury, however, is usually seen as a result of cardiopulmonary arrest, where the blood supply to the brain is inadequate for a period of time, resulting in the death of brain tissue. Reporter: What caused that brain injury? After doctors ran a battery of tests, they could narrow the possibilities. Is it possible that hid his was beaten and that caused brain injury? They did not see any evidence of fracture. Reporter: Dr. Arianne Lewis of New York university is a neurologist. She did not personally examine Otto. If he was hit on the left side of the brain, the tissues would be damaged on that side, but not the entire brain? Correct. If he was hit on the left side of the brain, then you would expect there to be a big injury on the left side, not something that was a global process throughout the entire brain. Reporter: The north Koreans had claimed that this was a case of botulism combined with a sleeping pill. But -- You say you didn't find any evidence whatsoever of botulism at all? That's correct. Yeah. Reporter: But Dr. Lewis says, even an autopsy, which Otto's parents decided to forego, would likely not have solved the mystery. There's nothing that can be looked at from a pathological perspective to say here's exactly the sequence of events that happened while over there. Reporter: Whatever the truth, many wonder why young pioneer tours would offer a trip like this. In a statement to us, they noted that none of the previous travellers who had been detained in North Korea had suffered such tragic finality. Still, they will no longer be organized tours for U.S. Citizens to North Korea and they expressed their deepest sympathies to the Warmbier family. For more answers, we sent an ABC producer here to China. Down this dark hallway, he found the young pioneer tour offices. Is this young pioneer tours. Reporter: The staff didn't exactly give him a five-star welcome. Hi. So, I'm with ABC news. Can we talk to you guys? No. Can we -- is there anyone available to talk to the camera? No. We released a statement and that's all we have to say. Are you seriously pushing me out? I just asked nicely -- who can I talk to? Is there anyone I can contact? Our media officer. We've been trying to contact him for the past few days.

This transcript has been automatically generated and may not be 100% accurate.