The Hamilton County coroner said only an external exam was performed on Mr Warmbier, who arrived home in a coma.
North Korea claims his coma was due to botulism and a sleeping pill, but the Warmbier family and doctors disagree.
Mr Warmbier, 22, was sentenced to 15 years' hard labour in March 2016 after being tried for attempting to steal a propaganda sign from a hotel.
The coroner's office in Cincinnati, Ohio, said in a statement: "No conclusions about the cause and manner of Mr Warmbier's death have been drawn at this time as there are additional medical records and imaging to review and people to interview.
"Our deepest sympathies are with the family and friends of Mr Warmbier at this time of their tragic loss."
More about: #OttoWarmbier
-1780923278.jpg&h=190&w=280&zc=1&q=100)
