Last November, in one of music’s more bizarre stories, two dolphins at a marine park in Lipperswil, Switzerland died after a rave festival was held just metres away from the dolphins’ enclosure at Connyland Zoo.

Wildlife experts warned that the intense techno music from the rave would cause the dolphins distress, but both the marine park and local planners – who gave permission for the rave – went ahead with the event regardless, resulting in the deaths of two dolphins, named Shadow and Chelmers, within five days of the techno gathering.

Though originally thought to be killed by the intense noise, a toxicology report has come to light that confirms that the pair of animals were instead killed by a drug overdose.

The blood tests and autopsy noted the presence of Buprenorphine, a heroin substitute used for treating recovering addicts, coursing though their systems. The heroin substitute was likely fed to the mammals by clubbers attending the weekend-long rave.

According to a Dutch marine biologist Cornelis van Elk, the tablets, once ingested would have suppressed the dolphins’ natural instincts to surface for air, causing them to drown.

Speaking to The Sun, the dolphin expert said “dolphins are conscious breathers, which means they actively decide when to come to the surface to breathe,” and that “drugging them with opiates causes this part of the brain to switch off, with fatal consequences”

Zookeepers speaking to the media told of the slow, grisly deaths of the pair of dolphins. Shadow was found dead by park officials, floating in the water; while marine park employee Nadja Gasser said Chelmers’ “death was very drawn out and painful…. it went on for over an hour,” she told local media.

“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.”

Even after administering adrenalin to the creature, Chelmers died after an hour, “it was horrendous. I have not been able to sleep since,” Gasser told local media.

Marine park officials originally had their own suspicions about the involvement of drugs, denying that noise was a factor in the animals’ death, which lead to the conducted autopsies and subsequent toxicology report.

Their original theory has now been confirmed.