Otto Warmbier, an American college student who was held prisoner in North Korea for 17 months, died of an unknown injury that occurred more than a year earlier, a coroner said Wednesday.

The coroner, Dr. Lakshmi Sammarco of Hamilton County, Ohio, acknowledged at a news conference that the exact cause of Warmbier's injuries and eventual death remains unknown. But she noted that a lack of oxygen and blood flow severely damaged his brain.

"We don't have enough information about what happened to Otto at that initial insult to draw any concrete conclusions," she said.

Warmbier, 22, was returned to the United States in June in a comatose state with severe brain damage. He died days later.

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Sammarco and a Sept. 11 coroner's report noted the cause of death as brain damage from a lack of oxygen caused by "an unknown injury more than a year prior to death."

Warmbier had been taken into custody in North Korea when he visited the country through a China-based tour company. North Korean officials alleged that the University of Virginia student attempted to steal a propaganda poster from a hotel.

Pyongyang has said that Warmbier's death was due to botulism and taking a sleeping pill, and insisted that the student was not mistreated or tortured.

Warmbier's parents, however, have alleged that their son was badly disfigured and tortured while he was imprisoned in North Korea. In an interview on Fox News this week, Fred and Cindy Warmbier said the country "destroyed" their son.

Warmbier's parents refused an autopsy for their son, and the coroner's report was based on a physical examination of the body and a CT scan.