The coroner said the 22-year-old, who had been sentenced to 15 years hard labor while visiting the reclusive country, had suffered brain damage caused by the lack of oxygen to the brain.
Coroner Lakshmi Sammarco could not say what caused the injury.
The revelations came a day after Warmbier's parents and Trump accused the reclusive regime of torturing the young man, who had been convicted of trying to steal a propaganda poster from a Pyongyang hotel.
The parents, in a series of TV interviews Tuesday, said their son showed signs of torture, including teeth that appeared to have been "rearranged," and hands and feet that were disfigured.
"They kidnapped Otto, they tortured him, they intentionally injured him. They are not victims, they are terrorists," Fred Warmbier said on the program "Fox and Friends."
After the airing of the interview, Trump for the first time accused North Korean leader Kim Jong-Un's regime of torturing Warmbier.
Trump called the parents' interview with Fox "great" and said: "Otto was tortured beyond belief by North Korea."
But Sammarco, who examined Warmbier's body after his death in June, said there was no clear evidence of physical torture - including no recently broken bones or damaged teeth.
"We don't know what happened to him. That's the bottom line," she said. "We're never going to know, unless the people who were there come forward and say, 'This is what happened to Otto.'"
Warmbier's body displayed only a few small scars, all but one of which could be traced to medical instruments, she said, adding that the Warmbiers' TV interviews had prompted her to publicly reveal her findings.
"They're grieving parents. I can't really make comments on their perceptions," she added.
Three Americans accused of various crimes against the state are behind bars in the North, which is engaged in a tense standoff with the Trump administration over its banned missile and nuclear weapons programs.
