The two dolphins died only five days apart from one another last year, raising questions once more about the handling of animals at the Conny-Land park.

The findings were contained in a report from the Institute of Veterinary Pathology at the University of Zurich. The report was presented by the Thurgauer prosecutor on Monday.

Criminal proceedings for negligent animal cruelty against the two prescribing vets are now being considered, Tages Anzeiger reported on Tuesday.

An earlier theory, that queried whether the two dolphins had been poisoned by a third party, has now been ruled out.

It is still unclear how the antibiotics caused the deaths. All drugs have side effects, and none have been specifically developed for use on dolphins, Paul Witzig, a Thurgau vet, told Neue Zürcher Zeitung. However, according to Kerstin Jurczynski, a vet at Duisburg Zoo, use of antibiotics to treat dolphins is not uncommon, and is often cited in veterinary literature.

This is not the first time the alarm has been raised against Conny-Land for its poor treatment of animals. During the 1990s, animal rights activists were particularly critical of the park. Before these two recent fatalities, Conny-Land lost two baby dolphins in 2000 and another 14-year-old dolphin in 2008.

A woman of 28 is also now under investigation for making certain threats against the park following the deaths of the two dolphins last year, NZZ reported on Tuesday.