Thai authorities continue to deny links between a spate of deaths in the northern city of Chiang Mai, and say they may never know what killed New Zealander Sarah Carter.
The Thai Public Health Ministry's Department of Disease has also rejected claims that insect control chemicals were to blame for Ms Carter's death.
Ms Carter, 23, became violently ill on February 3 while staying at the Downtown Inn in Chiang Mai and died a day later. Her two New Zealand friends and travelling companions, Emma Langlands and Amanda Eliason, also fell ill but recovered.
Retirees George and Eileen Everitt, from Britain, and Thai tour guide Waraporn Pungmahisiranon died within 16 days of Ms Carter, in adjoining rooms at the three-star hotel. All four are believed to have died after the sudden onset of severe heart conditions.
Authorities are now also investigating the deaths of two other foreign tourists, who fell ill in Chiang Mai about the same time and with similar symptoms.
Despite the shared symptoms, the Department of Disease said there were "few common traits" in the deaths. "Despite the best efforts of Thai authorities and international partners, a complete explanation for the cause of deaths may not be found for all cases," it said.
The department rejected the theory that the tourists died as a result of exposure to insect control chemical chlorpyrifos, often used to kill bedbugs.
Experts said chlorpyrifos generally emitted a strong odour, which was not reported by those who fell ill, and was not usually fatal unless ingested or inhaled in very large quantities.
An investigation by TV3's 60 Minutes, which screened this month, found trace elements of chlorpyrifos in Ms Carter's room, prompting calls for Thai authorities to investigate.
Ms Carter's father, Richard, said he thought Thai authorities were more concerned with disproving links between the deaths, and clearing the Downtown Inn, than in finding out what killed his daughter.
"It seems obvious to me, and obvious to everyone else in the world, that there are links ... I mean, there was a woman dying in the very next room while the girls were there."
Foreign Affairs Minister Murray McCully has said he is "less than convinced" by Thai claims that the deaths were a coincidence. A ministry spokeswoman said yesterday that, until cause of death was determined, it was impossible to rule out the chance that the deaths were connected.
"The New Zealand Government has strongly encouraged the Thai authorities to progress with their investigation."
- The Dominion Post
How soon will you visit the new war memorial park?Related story: War memorial park all but completed