An American student imprisoned in North Korea was returned to his family blind, deaf and howling incoherently on a stretcher, his parents have said, in their first interview since his death in June.

Fred and Cindy Warmbier’s son Otto, 22, had travelled to North Korea as a tourist in January 2016. He was arrested and accused of removing a poster from his hotel, and sentenced to 15 years in prison.

The student fell ill shortly after he was sentenced, doctors believe. The North Koreans said he suffered from botulism and had a violent response to a drug given to help him.

He was denied visits by Swedish embassy officials, acting on behalf of the US, and it was not until June, when the extent of his illness became clear, that he was freed.