I'm wondering if Otto Warmbier might have been given an OD of Sodium Pentothal ("truth serum") by the North Koreans while he was held there?   Too much of that stuff can produce comatose-like conditions.

In a book I read recently on North Korea, the author claimed that it is not uncommon for North Korean Public Security officials to administer it to foreigners resident there, in order to establish their bona fides. 

Warmbier should have just purchased a complete set of the works of Kim Il Sung for his trip souvenir!