A coroner could not determine what caused Otto Warmbier, the University of Virginia student who was imprisoned in North Korea, to suffer a dangerous loss of oxygen to his brain, according to a report released this week.

Warmbier returned to the United States in June after being in a coma for more than a year. The coroner in Hamilton County, Ohio, performed high-tech medical imaging tests and a physical examination of Warmbier’s body.

At some point more than a year ago, Warmbier’s brain was starved of oxygen, but many causes could explain that, and the report from the Hamilton County coroner could not point to a definitive cause.

“I’m not denying that he was tortured,” the coroner, Lakshmi Kode Sammarco, said. “I don’t know that he was tortured or not tortured.”

“We didn’t have physical evidence of torture other than the anoxic encephalopathy in the brain and the inexplicable scar on the top of the foot,” she said, using the medical term to describe oxygen deprivation in the brain.

Warmbier’s death — the loss of a young, scholarly, adventurous, much-loved University of Virginia student — touched off mourning, outrage and heightened tensions between the United States and North Korea this summer. It also led to many questions, most of which remain unanswered.

North Korea again denied Thursday that Warmbier was tortured, despite what his parents and President Trump have said in recent days.

The student’s parents, Fred and Cindy Warmbier, did not immediately respond to messages seeking comment, but earlier this week during an appearance on “Fox & Friends,” they described the North Koreans as “terrorists.”

“They kidnapped Otto,” Fred Warmbier said. “They tortured him. They intentionally injured him. They are not victims.”

Trump agreed that Warmbier was tortured, posting on social media:

“It’s really frustrating not knowing what happened to him,” Sammarco said.

The cause of death was “complications of chronic anoxic/ischemic encephalopathy due to unknown insult more than a year prior to death,” according to the coroner’s report.

It’s that “unknown insult” that may never be determined.

“What we needed to know was what caused his brain to be starved of oxygen,” the coroner said. That could have been because something stopped the blood flow to the brain or because Warmbier  stopped breathing for a significant amount of time, four to five minutes, depending on oxygen capacity.

The coroner mentioned many possible causes, including drowning, a stroke or medication. Nothing points to one cause being more likely than another, she said. 

The same thing with the scar on his foot — we consulted various specialists on skin injuries, they said the same thing. I asked, ‘Could it be this? Could it be this? Could it be this?’ and they said, ‘Yes.’”

Because the scar had healed, it was not possible to blame the injury on a single cause, she said.

The Warmbier family objected to a full physical autopsy, but a virtual autopsy and a physical examination did not reveal evidence of fractures, Sammarco said. A significant break would probably be detectable after 15 months as a healing, or healed, fracture.

The coroner’s report described multiple small scars and a large, more than 4-inch-long scar, about an inch and a half wide, on one foot.

“Something that all of us, all five doctors, thought was remarkable was that his skin was in very good, in fact excellent, condition, considering he had been bedridden for over a year,” Sammarco said: No bed sores, no significant breakdown of the skin, no pressure sores.

“That would require constant attention,” she said.

He didn’t look anything like he had in photos before he was imprisoned, but his muscles were not as atrophied as doctors would have expected them to be, either.

It was clear from the brain imaging that he had been unconscious for a very long time: A scan from April 2016 from North Korea revealed very significant brain damage at that time.

Warmbier was a healthy, athletic 21-year-old when he traveled with a tour group to North Korea on his way to a study abroad program in Hong Kong. He was not allowed to leave the country in January 2016. After a sham trial, he was sentenced to 15 years of hard labor and shortly after that, apparently was injured. His family heard nothing about him after March of that year — he was denied consular visits — until they were told in June 2017 that he had been in a coma for more than a year. He was medically evacuated home to Cincinnati.

On Thursday, the state-run broadcaster KCNA quoted North Korea’s foreign ministry spokesman as saying claims of torture were baseless slander designed to discredit the regime.

Fred Warmbier described to “Fox & Friends” walking up the steps of the plane when his son was medically evacuated to the United States after being imprisoned for more than a year and hearing “this howling, involuntary, inhuman sound. We weren’t really certain what it was. . . . Otto was on the stretcher across in the plane, and he was jerking violently, making these inhuman sounds.”

“As we looked at him and tried to comfort him, it looked like someone had taken a pair of pliers and rearranged his bottom teeth,” Fred Warmbier said.

“They destroyed him,” Cindy Warmbier added. She said she almost passed out but composed herself so she could go with him in the ambulance, assuring he would no longer be alone. Within two days, their son’s fever spiked to 104 degrees; there was a large scar on his right foot.

“Our image of Otto, as you know, was of someone just wonderful, beautiful inside and out. And to see how he came home was too much for us,” Cindy Warmbier said.

Sammarco said a forensic dentist found no evidence of trauma to the teeth. They were in a narrow part of the jaw and crowded, she said.

Patients with the extent of brain damage Warmbier suffered often make involuntary sounds and movements.

“The bottom line is we’re never going to know what happened to him,” Sammarco said, “unless someone who was there comes forward with the specifics . . .”

“This was a very difficult case,” Sammarco said. “We would normally do an autopsy in this case, but the family vehemently objected. . . . There was some suggestion there was a religious objection on the part of his mother.”

As a mother, she said, she can’t imagine their pain. “We have no words to express our sorrow for what they’re going through.”