Thailand's Phi Phi Islands are famous for the sun during the day and beach-side cocktail parties at night. This summer, two Canadian sisters set off for a rite-of-passage trip to the islands' white sands. They never came back.

Noemi, 25, and Audrey, 20, Belanger were found dead in their hotel room. Their deaths were among the latest in a series of mysterious deaths in Southeast Asia. Over the past few years, nearly a dozen young travelers, mostly Western women, have inexplicably died while traveling in the region.

The deaths have caught the attention of science writer Deborah Blum, who's written about them in Wired magazine. A poison expert, Blum tells weekends on All Things Considered host Guy Raz that a popular cocktail may hold a clue.

"One of the things that a lot of the tourists drink are called bucket drinks, where people will come to your table, or your chair, or where you're sitting in the sand, and they'll mix up a drink for you," Blum says.

It was at one of these after-dark beach parties where local police say the sisters ingested DEET, the chemical used in mosquito repellant. Thai investigators say it's a common ingredient in those bucket drinks because of its hallucinogenic effects, but Blum is skeptical about that claim.

"There is absolutely zero evidence that DEET is used in these cocktails on these beautiful Thai islands," she says.

Blum says the autopsies showed the sisters' deaths were gruesome and that the trauma to their bodies is inconsistent with DEET poisoning.

In late July, two women in their mid-20s were backpacking through Vietnam. Kari Bowerman, a 27-year-old American from Wisconsin, and Cathy Huynh, a 26-year-old Canadian, were on vacation from their jobs as English teachers in South Korea.

The companions arrived in Vietnam one day, and the next day they were in the hospital, Blum says. "They were, both of them, suffering from symptoms of what looked like acute poising."

Huynh was discharged from the hospital, but returned later that night to visit Bowerman — only to find out she had died. Huynh herself died in the hospital two days later.

A pesticide called chlorpyrifos is being blamed for the women's deaths. Though commonly used to treat bedbug infestations in Asian countries, it's banned for use in residential buildings in the United States because of its link to developmental problems in children.

Blum says the theory that Bowerman and Huynh died from careless fumigation at their hotel doesn't add up.

"If you're doing widespread fumigation, it's not like you're just doing one room," she says. Why didn't hotel workers or other guests die?

Rather, Blum thinks the deaths of all these women may have been deliberate. In the case of the Belangers, she says, "why would these two young girls ... be the only ones who die from a poisoned cocktail? That doesn't sound to me like you're randomly drinking the popular beach drink. That sounds to me like someone picked you out."

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