Two dolphins at a Swiss zoo died after ingesting the heroin substitute drug buprenorphin some time after a "rave" near their habitat, a post-mortem has shown.
The mammals died at Connyland zoo, Lipperswil after a weekend-long festival held near the dolphinarium.
A toxicology report released over the weekend showed that the two ingested buprenorphin, a drug used as a heroin substitute, which caused Shadow and Chelmers to die slowly and painfully.
The party was organised by the Connyland park management, however they deny any wrongdoing in the treatment of the animals. The park's owner and general manager, Roberto Gasser, contacted the Huffington Post UK denying that the party had anything to do with the dolphins' deaths, instead blaming a "planned strike" by animal activists.
Their death was initially blamed on the zoo's vets, who were assumed to have given the dolphins incorrect medicine before the toxicology report finally revealed the real cause of the dolphins' death.
It is not clear who gave the dolphins the drug or when, and whether it was a deliberate act.
The dolphins were bought for the zoo by its founder Nadia Conny, who asked Princess Stephanie of Monaco to be the dolphins' godmother.
Princess Stephanie of Monaco met the dolphins back in 2004
Dutch marine biologist Cornelis van Elk added that the drug is incredibly dangerous for sea mammals.
"Opiates are extremely dangerous for underwater mammals.
"Even when sleeping - there is part of the brain that automatically controls the breathing instinct in the same way as it does for people when asleep," van Elk explained.
"Drugging them with opiates could well cause this part of the brain to switch off with fatal consequences."
Their keeper, Nadja Gasser, said that they dolphins died an awful death. Shadow died very soon after the event, with Chelmers dying five days later.
"The death was very drawn out and painful. The death went on for over an hour. It was horrendous. I have not been able to sleep since," Gasser said.
"He [Chelmers] was drifting under the water and was clearly in trouble and so we jumped into the water. We tried to hold him. He was shaking all over and was foaming at the mouth.
"Eventually we got him out of the water. His tongue was hanging out. He could hardly breath. He was given adrenalin, but it didn't help. After an hour the dolphin died."
Correction: This article previously stated that the drug was almost certainly given to the animals at the party, however the park's owners have since disputed that this is the case