An American student, who had been imprisoned in North Korea for 17 months, had died from lack of oxygen and blood to the brain, a coroner in Ohio in the United States said on Wednesday, according to a Reuters report. Otto Warmbier had died on June 19 at a hospital in Cincinnati, days after North Korea had released him in a comatose state.

At a news conference, Hamilton County Coroner Dr Lakshmi Sammarco said the 22-year-old had died because of an unknown injury he had sustained more than a year ago. “We don’t know what happened to him, and that’s the bottom line,” Sammarco said.

The coroner’s report said there were complications of chronic deficiency of oxygen and blood supply to the brain. Only an external examination of the body, and not a full autopsy, had been conducted at the request of Warmbier’s family. Sammarco said there was no clear evidence of physical torture, including no recently broken bones or damaged teeth, Al Jazeera reported.

His comments contradict the statements made by Warmbier’s parents in a TV interview with Fox on Tuesday. They said North Korea had “systematically tortured” their son after he was jailed.

Warmbier’s parents have yet to comment on the coroner’s report.

What happened to Warmbier?

In March 2016, North Korea had sentenced Warmbier to 15 years of hard labour for stealing a propaganda sign from a hotel. The University of Virginia student was convicted of “subversive activities” by the country’s Supreme Court. The US had appealed for his release saying he would not have faced the same charges for the offence anywhere else in the world.