Two dolphins that were found dead at a Swiss zoo in November were perhaps killed by toxic drugs fed to them by the ravers and not by antibiotic overdose or noise as was previously alleged, according to the latest report.

A leaked toxicology report cited by the Daily Telegraph, says tests conducted by the forensic institute in St Gallen have found contents of Buprenorphin (a heroin substitute) in dolphins’ urine, confirming the role of toxic drug in their death.

The dolphins, called Shadow and Chelmers, were found dead in a mammal pool located just a few yards from the amusement park in Lipperswill where the controversial two-day techno party was held in November 2011.

The report suggests that conceivably a few clubbers had fed the dolphins an illegal recreational heroin-like substance that led to their death.

Earlier, the prosecutors blamed antibiotic overdose that resulted in brain damage and shattering music levels for the death. However, the latest report has raised new apprehensions.

The zookeeper of Connyland, where the rave was held despite warnings about the dangers to the wildlife, has said that the dolphins died a “drawn out and painful” death.

Nadja Gaser added: “The death went on for over an hour. It was horrendous.”

Gaser revealed that both the animals died the same way. “When we went to start the dolphin training we noticed the same thing that had happened to Shadow was happening with Chelmers,” he told the Daily Telegraph.

The zookeeper described how all their attempts of saving the mammal went in vain as they died one after another within less than a week of each other. “He 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,” he said. “Eventually we got him out of the water. His tongue was hanging out. He could hardly breathe.”

“He was given adrenalin, but it didn't help….After an hour the dolphin died,” Gaser told the daily.

Previously, the prosecutors had blamed the Zoo authorities for the appalling casualty, notwithstanding the allegation against the rave-goers.

However, Dutch marine biologist and dolphin expert Corneils van Elk says that the chances of them being given the drug as antibiotic treatment by zoo vets were less.

“Opiates are extremely dangerous for underwater mammals and would never be used in any legitimate treatment,” he asserted.

“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,” Elk explained the reason behind their death.

“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,” he told the paper.

Though as many as seven dolphins have died in the same zoo in three years Connyland has continually denied any foul play.