The UN Special Rapporteur for human rights in North Korea Tomás Ojea Quintana on Friday criticized the DPRK’s treatment of American student Otto Warmbier who was returned to the U.S. in a coma 18 months after his arrest.

Quintana called on the DPRK to provide a clear explanation on how Warmbier came to be in a coma. So far, the North Korean authorities claimed he contracted botulism and then took a sleeping pill, though doctors in the U.S. said they could find no evidence of the rare disease.

“While I welcome the news of Mr Warmbier’s release, I am very concerned