Shadow and Chelmers, two dolphins living in the Connyland marine park in Lipperswil, Switzerland died after ingesting heroin, Australian News.com reports.

The dolphins died last November after a rave was held at the zoo. At first experts believed the two dolphins died due to the loud rave music.

According to The Mirror, a new toxicology report leaked to Swiss media found the heroin substitute Buprenorphin was detected in the animals urine.

Dutch marine biologist Cornelis van Elk told the Mirror, the drug caused the dolphin's brain to shut off their natural instinct to come up for air.

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"Opiates are extremely dangerous for underwater mammals and would never be used in any legitimate treatment," said Elk. "The reason is that dolphins are conscious breathers which means they actively decide when to come to the surface to breathe, for which they need to be awake. 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. Drugging them with opiates could well cause this part of the brain to switch off with fatal consequences."

Connyland zookeeper Nadja Gasser told The Sun, Chelmers was found dead immediately following the rave. Shadow was found to be exhibiting strange behavior. Gasser explained Shadow's death was nothing short of horrific. “The death went on for over an hour. It was horrendous. I have not been able to sleep since.”

News.com said Connyland has denied any wrongdoing by green lighting the rave.

Connyland spokesman Erich Brandenberger told the Mirror, they will be following up with prosecutors on why they originally blamed zoo vets instead of investigating if the animals were poisoned.

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