The two dolphins (not pictured) died from an overdose of the opiate Buprenorphine, which may have shut down the brain mechanism that tells dolphins when to surface for air.

Kike Calvo via AP Images

ast November, two dolphins at a Swiss zoo endured slow, painful deaths after the facility hosted a weekend rave. Initially, puzzled officials blamed the animals' fates on everything from blaring music to vet negligence. But new toxicology reports reveal that the mammals died of a drug overdose. Here, a brief guide to this tragic story:

What happened to the dolphins?
Shadow and Chelmers of the Connyland Marine Park in Switzerland died mysteriously, and within five days of each other, following a weekend-long rave. "Animal-rights activists originally blamed the deaths on the event's techno music," says Courtney Garcia at MSNBC, suggesting that the thumping bass "disoriented and distressed" the sonar-dependent animals and weakened their immune systems. Zoo officials also suspected the facility's vets of feeding the dolphins incorrect dosages of antibiotics, and considered pressing negligence charges. But, now, new toxicology results confirm that the culprit was a heroin substitute called Buprenorphine, which is typically used to wean addicts off the drug. 

How did dolphins get these drugs?
Experts are speculating that one of the thousands of party-goers dumped the Buprenorphine in the tank accidentally, or perhaps intentionally, amused by the prospect of doping up dolphins. 

Why was this so deadly?
The drug likely played lethal tricks on the dolphins' brains, says Tim Newcomb at TIME, "switching off the monitor telling them they need to surface for air." Shadow was reportedly found dead after the rave. At some point after that, Connyland dolphin keeper Nadja Gasser noticed Chelmers drifting underwater unnaturally. "We tried to hold him," she said. "He was shaking all over and was foaming at the mouth," and his death was "very drawn out and painful." 

Why would anyone hold a rave at a zoo?
That's what animal rights activists want to know. But Connyland owner Roberto Gasser insists that his zoo has rented out a "sound-proof chateau" near the dolphin enclosure every weekend for the last ten years, and that a vet was present during this particular rave. Plus, he says, these dolphins "died two weeks after the party, so the party had nothing to do with it at all." But Nadja Gasser disputes that account, saying the dolphins died much sooner after the rave.

Sources: The Huffington Post (2), MSNBC, Sun, TIME