The Phi Phi Islands sit off the western coast of Thailand, floating like jewels in a turquoise sea, a picture-perfect image of a tropical getaway. Director Danny Boyle filmed his 2000 psychothriller, The Beach, on the largest of those islands and if you know the movie, you know, despite the gem-like setting the story ends badly.

They say, though, that the movie put the largest of the islands, Ko Phi Phi Don, on the map as a tourist getaway, a reasonably priced home to glittering beaches and unlimited partying. And that's undoubtedly what drew two young sisters from a small Canadian village, just north of the Maine border, to travel there for a summer break from their university studies.

Noemi Belanger was 26 and her sister, Audrey, 20, when they planned the June vacation. Both sisters lived in their hometown of Pohenegamook, Quebec. Did I mention that it was small, the kind of place where people know each other, stay close? The population is about 3,000 and both girls worked for their father, Carl, in his grocery store before starting university classes. They were happy girls, friendly, residents say, involved in their community, helping out at the local library, at public beaches.

This summer, they were ready to fly a little, indulge in a splashy vacation. So they saved their money and flew to Thailand in June, went to visit the Phi Phi Islands. And there, as a flood of mid-June news stories made obvious, things went very wrong. Very, very wrong.

The stories were puzzled, horrified. A story in Canada's National Post described a hotel maid finding the sisters dead in their room, with lesions tracked across their bodies, their fingernails and toenails turned an odd grayish blue. They were huddled in their beds, relayed the Global Post, smeared with vomit and blood.

Rumors flew of an exotic poison, of a lurking killer. Dismissive statements from the police added to the sense of mystery. "We found many kinds of over-the-counter-drugs, including ibuprofen, which can cause serious effects on the stomach," one investigator said, sounding as if packing painkillers was the real problem. Mysterious poison deaths of tourists visiting the Phi Phi Islands were recalled: the 2009 death of a Seattle woman, still unsolved today. The similar and also unexplained death of a 22-year-old woman from Norway the same year. An odd cluster of deaths in another Thai city during winter of last year, including a 23-year-old woman from New Zealand. The conspiracy theories expandedto includethe unexplained deaths of two young women in Vietnam this summer. "Is this a cover-up?" asked a letter writerin the Bangkok Post after the police went on from the ibuprofen theory to one that involved food poisoning.

And not just any food poisoning. A leak from the investigationsuggested that detectives were considering the possibility that the sisters had dined - somewhere - on either poisonous mushrooms or blowfish, sometimes called pufferfish or fugu. The fishes are considered a delicacy but they must be carefully prepared to exclude any contact with the liver or other internal organs, which contain an exceptionally potent neurotoxin calledtetrodotoxin.

Neither of these suggestions, though, were an ideal match for the described symptoms. Tetrodotoxin is most famous for its ability to induce a corpse-like paralysis in victims; they may remain alert but unable to move or communicate, gradually suffocating as the lungs fail. Poisonous mushrooms tend to kill by gradually destroying the liver. As quickly as the suggestion was floated it seemed to disappear, leaving the questions to further simmer over the summer.

Until last week, when a preliminary autopsy report was announced, which apparently indicated a toxic level of exposure. According to news reports, toxicologists in Thailand now believed that the two sisters had been drinking a popular local cocktail that contains Coca-Cola, cough syrup, ground up leaves from the kratom tree, and the well-known mosquito repellent DEET and is admired for its hallucinogenic qualities. In their case, apparently, too much DEET had ended up in the drink.

Or as the tourism-focused island paper, Phuket Wan, wrote following the announcement:

Phi Phi is renowed as a rites-of-passage destination for 20-somethings and it transforms from a haven for day-trippers in the sunshine to a less beguiling island party after dark. Alcohol is just one of the many ingredients that Phi Phi's party people mix in their buckets. Each bucket is a concoction of all kinds of juices and substances that are mixed into containers of various sizes and usually sucked through straws all night long.

It's a nicely sinister portrait of cocktails in the Phi Phi islands. Still my first reaction was a kind of "DEET, really?" skepticism. We're not talking about anything like tetrodotoxin here; this is a compound we routinely spray all over ourselves on camping trips and summer hikes. Our Environmental Protection Agency estimates that about 30 percent of the U.S. population uses a DEET-infused product every year. Plenty of us have accidentally swallowed a little during an over-enthusiastic assault on mosquitoes without getting sick (including myself). Not that you'd want to take it by the tumbler, of course. But it's reasonable to ask whether it would take a tumbler to kill you

The short answer, yes, pretty close to that. DEET, by the way, stands for N.N-Diethyl-meta-toluamide, which is basically chemist-code for a formula that includes the familiar elements carbon, hydrogen, nitrogen and oxygen. It apparently works as a repellent by disrupting insect olfaction-detection systems. And an EPA analysis found that it is slightly toxic to birds, fish, and aquatic invertebrates and has"very low toxicity potential" in mammals, such as ourselves.

So it's not surprising that the Agency for Toxic Substances and Disease Registry reportsthat people have committed suicide with the repellent but only by drinking full "bottles of DEET" along with quantities of alcohol. In other words, evidence is that it would take that tumbler full to kill you. I also looked at the other ingredients in the suspect cocktail, except for the Coca-Cola (which hasn't contained cocaine for more than a century). The codeine in cough syrup could, in a high enough amount, add to a sleepy buzz. And kratom - while known to be hallucinogenic - can also bring on a numbing lethargy in too high a dose. It is generally, though, considered to be most risky for its addictive qualities than for its acute toxicity issues.

Which brings us back to the DEET theory of death. And that requires someone to pour a ridiculously large quantity of this pale yellowish liquid into a drink served to two sisters from Canada. Could someone be that careless? Sure, especially if they were enjoying the island brew themselves. Still, only the Belanger sisters died after that night on the beach; under this theory only one overtoxic cocktail was served. And that does raise a few other questions. For instance, why - as you may have noticed from my fatality list - is it mostly young women who are dying of mysterious chemical poisonings in a tropical paradise?

Even Phuket Wan (which seems remarkably tough-minded for a publication focused on tourism) seems unconvinced by the mosquito repellent hypothesis, noting that it would be unusual for only two people to be poisoned by a shared bucket drink.

Could it be a cover up, the paper asked, for a heavy-handed use of insecticide in the sisters' room? Insecticides have been suspected in some of the other deaths. Could it be that island authorities were trying to hide the existence of a killer who was deliberately spiking drinks? Or, slightly less creepily, that the women had been killed by excessive use of insecticides by hotel management and that authorities were moving to protect reputations? "All options remain open," the paper warned, until the authorities produce evidence of a much more meticulous investigation.

And, yes, you'll find me in the "options remain open" camp as well. It may well be that this is as simple as it sounds, two trusting travelers from rural Canada drinking an untrustworthy bar drink. Still, at the moment, if I felt a sudden urge to go party in the Phi Phi islands, you would find me insisting on a nicely capped container - and, I think, opening that bottle myself.

Which, frankly, makes good sense most of the time anyway.

Images: 1) Phi Phi Island bay, byiamdave/Flickr 2) Handout photo/Phuket News