Chicago: An American student whose mysterious death occurred soon after he returned from 17 months in jail in North Korea, died from lack of oxygen and blood to the brain, an Ohio coroner said on Wednesday.

Otto Warmbier's death on June 19 was due to an unknown injury that occurred more than a year before his death, Hamilton County Coroner Lakshmi Sammarco said at a news conference.

"We don't know what happened to him and that's the bottom line," Dr Sammarco said.

Warmbier's parents could not be reached for comment on the coroner's report.