This summer, four young women set off on vacations in Southeast Asia. Here's what they had in common: They were all from North America; they were all in their 20s; they were all pretty, bright, adventurous. And one more commonality: They all died.

Two of these deaths occurred in June in Thailand, two in June in Vietnam. All four women were diagnosed with the symptoms of acute poisoning. And while some explanations have been offered by the authorities, these have been either vague, improbable (see my recent post on the deaths in Thailand) or opaque (see CNN's Friday story on the deaths in Vietnam). My favorite statement is one from the Thai police declaring that it could be "months before official results are revealed if ever." (Emphasis mine).

If ever? What kind of a police response is that? Does it mean that investigators know something they don't want to tell? Or that they don't have a clue? It's no wonder that the rumor mills are spinning stories of murder, of a serial killer stalking female tourists in Southeast Asia, of a police cover-up to protect the valued tourist industry. The serial killer idea, of course, builds on earlier mysteries: 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 other theory circulating is that the police are covering up the careless use of insecticides by Asian hotels; an explanation denied, of course, by the hotel industry. It doesn't explain, of course, why most of these deaths involve females in their 20s. But there's some support for it from an independent investigation into the 2011 death of New Zealander Sarah Carter.

Carter was staying at a hotel in Chiang Mai, Thailand, when she died (along with five other tourists). The police blamed a coincidental outbreak of food poisoning. But Carter's family turned over tissue samples to investigative journalists from a New Zealand television station. The resulting laboratory analysis reportedly found traces of an old-time organophosphate pesticide called chlorpyrifos.

Sarah Carter Sarah Carter

This insecticide has been around since the mid-1960s. It's a Dow Chemical Company product sold in the United States under the tradename Dursban. Like all organophosphate pesticides, it's highly effective due to its action on the the nervous system Although it's only considered moderately toxic to humans, chlorpyrifos is linked to neurological effects and can pose developmental risks to children. Although still widely used in agriculture, it's no longer registered for use in residential settings in the U.S. But Dow does market it for such uses in developing countries, leading to suspicions that it had been surreptitiously sprayed in Carter's hotel to treat for bed bugs.

The problem with this theory – as with so many of the theories floated in the case of these Asian tourist deaths – is that it doesn't hold up well under scrutiny. Autopsies reportedly found myocarditis (put simply, an inflammation of the heart muscle) in some of the dead tourists but the classic symptoms of chlorpyrifos poisoning tend to be those of classic neurotoxicity, starting with dizziness and loss of coordination, ending with a gradual shutdown of heart and lungs. As Thailand's tourism-focused publication, Phuket Wan, reported the investigation into the death of Sarah Carter and others in 2011 simply ended in mystery.

Cathy Huynh and Kari Bowerman Cathy Huynh and Kari Bowerman

The possibility of chlorpyrifos or some other insecticide poisoning has also been raised in this summer's deaths of American Karin Bowerman, 27, of Lake Geneva, Wisconsin, and Canadian Cathy Huynh, 26, of Hamilton, Ontario. The two friends - both working as English language teachers in South Korea - were backpacking in Vietnam, when they were admitted to a hospital in the beach town of Nha Trang in late July, suffering from vomiting, dehydration and difficulty breathing. Bowerman died that day; Huynh two days later.

But so far no information seems to be available about how they might have been exposed to that or any poison. "No police report. No hospital report. No nothing," Bowerman's sister, Jennifer Jacques told CNN in a fury of frustration. The lack of information has led friends of Bowerman's from Winona State University in Minnesota, where she graduated, to launch a letter writing campaign to U.S. officials, begging for help.

Jill St. Onge Jill St. Onge

Another group of friends has launched a Facebook page, Protected Travels, which serves as both an archive of such mysterious deaths and a resource for travelers. Among the unexplained deaths you'll find on the page is that of Seattle's Jill St. Onge, who died of an apparent - and still unexplained - poisoning while visiting Thailand's Phi Phi Islands in 2009.

In the case, I wrote about earlier, the deaths of Canadians Noemi and Audrey Belanger also in the Phi Phi Islands in June, the police have posited a string of theories, beginning with food poisoning and ending, for the moment, with a toxic bar drink. The Belanger sisters, aged 26 and 20, were vacationing in the islands when they were found dead in their hotel room on June 15, reportedly smeared with vomit and feces and marked by bloody skin lesions and blue-black fingernails.

In early September, investigators announced that the two young women had been killed by the mosquito repellent DEET, which they explained was often mixed into beach cocktails, known as bucket drinks. As I pointed out in my earlier post, this would be a good explanation if DEET were really all that poisonous. But, in fact, even a basic check of the toxicology databases will tell you that it isn't, that the U.S. Environmental Protection Agency classifies it with "low level" toxicity for mammals.

Is it entirely benign? Absolutely not. There were some studies following the Gulf War, for instance, that found that DEET, in combination with other pesticides, could induce some dismaying neurological symptoms. Can it kill people? Yes, people have successfully committed suicide by mixing one more or more bottles of pure DEET with pure alcohol. Do those deaths look like the bloody messy ones of Belanger sisters? No. DEET's effects are, again, primarily neurotoxic.

There was, in fact, an immediate skeptical outcry at Teakdoor, a listserv for expatriates living in Thailand. As one Teakdoor member wrote me off-list: "I have lived in Thailand for years and I have been immersed in all-things Thailand and I have never heard for mixing DEET into a bucket drink. Google 'bucket drink' and see if you can find anything regarding DEET or insecticide-in-bucket-drinks that is not related to the Belanger sisters story."

I did just that and I could not find a single story UNTIL I found this one, which made it clear that DEET is not regularly mixed into Phi Phi Island cocktails. The standard recipe instead is kratom leaf extract (a slightly hallucinogenic compound derived from a regional tree), cough syrup (containing codeine, I presume), Coca-Cola and ice, and is known as a 4 x 100. So even if we disregard the question of improbable symptoms, we're left with these questions: If DEET is not a standard ingredient in a bucket cocktail, if it's only lethal in a deliberately high dose, if the Belanger sisters were the only tourists poisoned that night, then who killed them? If this is murder - covered-up to real - let's not forget that there are other compounds that more neatly fit autopsy report, ranging from the solvent toluene to the date-rape drug, GHB.

I'm not owed answers regarding the deaths of these young women. But their families deserve an honest response. And women traveling in Southeast Asia are owed more concern for their safety that this. And if the authorities in Southeast Asia want to sound something other than indifferent, I can promise you, that "if ever" is never going to be good enough.

Images: Belanger sisters, handout photo/ Herald Sun 2) Sarah Carter, handout photo/Dominion Post 3)Huynh and Bowerman, handout photo/Changrai Times. 4) St. Onge, memorial page.