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  (Photo: Warner Bros.)
  Nathan Gamble plays Sawyer Nelson in "Dolphin Tale," a movie that recounts a boy's struggles to save an injured dolphin.

Dolphins were killed by Swiss drugs that suppressed their instinct to breathe, drowning them. Now, prosecutors say the drugs were introduced to the animals during a zoo rave- a partygoer may have given the dolphins a heroin substitute.

The dolphins killed by Swiss drugs were named Shadow and Chelmers, and seemed to be fine before the November rave was scheduled at Connyland marine park in Lipperswil, Switzerland. Toxicology reports taken by a forensics institute at St. Gallen showed Buprenorphine in their urine, which prevented their instincts to surface for air.

"The reason is that dolphins are conscious breathers, which means they actively decide when to come to the surface to breathe," Dutch marine biologist and dolphin expert Cornelis van Elk told the U.K.'s Sun. "Drugging them with opiates causes this part of the brain to switch off, with fatal consequences."

Organizers at the park initially rented out space close to the dolphins' habitats for the rave, despite being told by animal activists that it could be dangerous for the mammals. The activists were concerned that the loud techno could damage the Shadow and Chelmers' finely-tuned sonar and echolocation abilities, or even impair their immune system.

The activists' dire warnings could be the reason prosecutors initially pursued that angle, ignoring the possibility that the animals could have been drugged at the rave. They first pegged Connyland zookeeper Nadja Gasser, but he insisted he was innocent.

"Opiates are extremely dangerous for underwater mammals and would never be used in any legitimate treatment," said van Elk.

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Furthermore, Gasser was present for the dolphins' deaths- an experience he called "horrendous."

"[Shadow] 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," said the zookeeper. "His tongue was hanging out. He could hardly breathe."

The death of the animal took an emotional toll on Gasser, who was there until the end.

"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," he admitted.

It is unknown if anyone was charged for the negligence regarding the creatures.