What could connect the dead of tourists on Ko Phi Phi and in Chiang Mai?

Update from 11.10.2012:
It remains a mistery, what killed Canadian sisters Noémi Bélanger, (25), and Audrey Bélanger, (20) on Phi Phi Island. Canadian reporters have reported that the girls had been killed as a result of ingesting DEET, a chemical compound usually applied to the skin to dissuade Mosquitoes from biting. This was linked to the habit of tourists, to enjoy their drinks from buckets. And some contain cough syrup, Coke, DEET and ground up kratom leaves, which are a mild narcotic indigenous to Thailand. This story was told by phuketwan.com. But now pattaya103.com reports, that Canadian Pathologists have confirmed, in a coroner’s report, that the levels of DEET, found in the bodies of the two sisters, was not the cause of their death. So the uncertainity is still there: what happened top the two sisters?


Update from 22.6.2012:
Police are looking for two Portuguese men last seen with the two holidaying Canadian sisters. CCTV footage from cameras at Phi Phi Palms Residence Hotel captured a male foreigner escorting the sisters back to their hotel room. Read more on Bangkok Post. The Superintendent of Krabi Town polce, Colonel Jamroon Rumrom, said police were now awaiting the results of the autopsy performed on the sisters in Bangkok earlier this week. About the men on the security cameras he said "they had no connection to the deaths of the women". A nurse at Phi Phi hospital told Phuketwan yesterday that there were cases of food poisoning on the popular holiday island every day. Read more on Phuket Wan.


Post from june 2012: Last week two Canadian sisters were found dead in their room at the Phi Phi Palm Residence Hotel at Koh Phi Phi. Until now its not cleared by autopsy, what caused their deads. It seems they died by poisoning. But what kind of poisoning? The blog off-the-path.com has a possible explanation, that should be considered, when staying in certain hotels in Thailand. It remembers that in 2011 seven people in Chiang Mai died after staying at the Down Town Inn in Chiang Mai. Also their deaths were related to poisoning. Tests then showed small traces of an insecticide called chlorpyrifos (CY) inside a room – a chemical that is used to get rid of bed bugs and has been banned from indoor use in many countries. Read more on off-the-path.com and on dailymail.co.uk.
Read reviews of Palm Residence Hotel.
phuketwan.com has a report by Alan Morison. He points out, that in 2009 two young women took sick on Phi Phi and died in mysterious circumstances. No answers were ever found, what killed American Jill St Onge, 27, and Norwegian Julie Bergheim, 22, on Phi Phi. Thew were staying at Laleena guesthouse. Read more. At the same time two other persons got ill, but survived. Read earlier reports. Jill and Julie died not the same day, as Jill's fiancee ryankells wrote.


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Koh Phi Phi Beaches and Resorts