Thurgau’s cantonal prosecutor has confimed the results of an investigation carried out by Zurich University’s veterinary department.
The prosecutor says foul play from a third party has been ruled out.
Investigations are still ongoing to see whether the two vets who prescribed the drug are legally liable.
The death of the two dolphins at the park in Lipperswill sparked national and international dismay.
In Sweden over 1,000 signatures were collected calling for the park to be closed down.
Parliament already received a petition from OceanCare in November demanding the closure of Conny-Land and a ban on dolphin imports.