INVESTIGATIONS into a ''cluster'' of mysterious deaths of tourists in Chiang Mai have so far produced no clear patterns, an official update reveals.

The riddle of what killed New Zealander Sarah Carter and five others remains, and inquiries are reported to be continuing.

The latest official account rejects any link involving a seventh victim and makes the point that there is no sure connection between all six 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,'' the report says, noting that toxicology investigations can take time.

Toxicologists from seven countries or international organisations are involved in continuing investigations, says the Department of Disease Control in Thailand's Ministry of Public Health.

Interestingly, although the deaths of two young female tourists on Phi Phi in 2009 are not part of this investigation, the Chiang Mai cases update reports that the bug-killer chemical ''chlorpyrifos generally emits a strong odor which was not noted by the two surviving women.''

However, American Ryan Kells, one of the survivors of the incident on Phi Phi that killed his fiancee Jill St John and Norwegian Julie Bergheim in adjoining rooms at the Laleena Guesthouse two years ago, has always said he noted a strong chemical smell when entering their room.

The chances of solving the mystery of the Phi Phi deaths is slim but in the Chiang Mai cases, it appears an effort is at least being mounted to explain six deaths and salvage some of the popular destination's tourism reputation.

Here's what the latest official report says:

International toxicologists consulted in Chiang Mai tourist deaths - Update 3

23 May 2011 - Thai public health authorities have enlisted the assistance of international toxicology experts this month to determine whether toxins or chemical agents might be involved in the deaths of five foreign tourists and a Thai guide and the associated illnesses of three other individuals in Chiang Mai earlier this year.

The Department of Disease Control, Ministry of Public Health (MOPH) and the Chiang Mai provincial authorities, enlisted the help of these expert toxicologists as part of the on-going investigation into the deaths that occurred between January and February this year. The toxicologists are from Australia, India, Sri Lanka, the United Kingdom, the United States, the World Health Organisation (WHO) and the US Centers for Disease Control and Prevention (US CDC).

This consultation followed an earlier meeting of experts at the Department of Disease Control held on 10 May 2011 with the Chiang Mai provincial authorities to review information gathered over the past few months. This meeting involved clinicians, epidemiologists, laboratory specialists, pathologists, toxicologists and police as well as the WHO and the US CDC. Subsequently, representatives of relevant consulates were briefed on the progress of the investigation.

The toxicology experts have reviewed the available evidence and discussed several possible causes involving toxins and chemical agents as a cause of death. The toxicologists also discussed the suggestion made by the New Zealand TV3 '60 Minutes' program that chlorpyrifos (used as an insecticide) was responsible for the death of one woman from New Zealand and the illness of her two traveling companions.

While a cause has not been clearly identified to date, experts recognised that chlorpyrifos generally emits a strong odor which was not noted by the two surviving women. They also noted that chlorpyrifos is not well absorbed by the skin nor would it cause rapid illness or death in a healthy adult unless it was ingested or inhaled in very high quantities. Furthermore, the details of the clinical and laboratory investigation do not support chlorpyrifos as a cause.

The toxicologists are also considering the possibility that other chemical agents including those found in pest control products might cause signs, symptoms and laboratory findings consistent with the features seen. Experts are also currently working to identify the most suitable laboratories to carry out specific tests for selected pest control ingredients in the remaining samples.

Concurrently, environmental and toxicology assessment involving experts and laboratories from the Ministry of Agriculture; the Department of Health, MOPH; Ramathibodi Poison Center; the Faculty of Agriculture, Chiang Mai University is underway at the hotel where four persons died.

The initial assessment of overnight room ventilation in April found adequate air ventilation with normal levels of nitrogen dioxide and sulfur dioxide; and no evidence of carbon monoxide accumulation. A parallel investigation has also been undertaken by local police.

While Thai authorities and international partners are actively pursuing the hypothesis that these six deaths might be linked to a similar cause, the investigation so far reveals few common traits except that:

..All occurred in Chiang Mai between 11 January and 19 February

..Six of the nine reported people affected lodged at one hotel (three were at two different hotels)

..The first six people identified are young (aged 23-29) female non-Thai nationals

The people who died and fell ill are being examined in three groups based on the hotels they stayed in, as follows:

1) One American woman (age 33) who died on 11 January and her Canadian colleague (age 29) who fell ill

2) One French woman (age 25) who died on 19 January

3) a) One Thai woman (age 47) who died on 3 February

b) One New Zealand woman (age 23) who died 6 February and her two traveling companions (both age 23) who both fell ill

c) A couple from the United Kingdom - one man (78) and woman (74) who both died on February 19

Based on findings currently available, there is insufficient evidence to link the illnesses and deaths among these three groups.

The American, Canadian and French woman had no activities in connection with other hotels and other cases. The French women also began to fall ill few days before arrival in Chiang Mai.

There have been reports that a Canadian man who died in January may have used the swimming pool of the hotel where the Thai woman, the New Zealand women and the United Kingdom couple stayed (hotel number 3 above).

However it has not been possible to confirm these reports or even to establish the day when this visit was supposed to have occurred. In addition, the available evidence from medical records and autopsy by forensic expert do not currently support linkage with other deaths. Therefore the death of this man has not been included in the cluster.

It was also initially believed on the basis of clinical findings that myocarditis (inflammation of the heart muscle) was a common factor in a number of deaths. However, although it is clear that heart muscle damage occurred in these cases, further investigation of clinical, laboratory and histopathology records, has only established clear evidence of myocarditis in the case of the French woman.

It is partly this evidence that has resulted in the scope of the investigation being widened to look for other causes of cardiac damage (that can mimic the clinical appearance of myocarditis), including toxins and chemical agents.

A final explanation of the most likely cause of the event(s), including exact cause of illnesses and deaths, and any important related factors, will be made based upon the synthesis of evidence from clinical, epidemiologic, laboratory, environmental and forensic investigations.

It is important to highlight that a complex investigation such as this which also includes international laboratory analyses takes time. Any new toxicology results are also likely to take time if specimens need to be shipped to specialist laboratories.

It is also possible that cases may not all be linked, and may instead be due to different causes. 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.

The Thai Ministry of Public Health will provide information as it becomes available to the families, the international community and the media, and will provide regular updates on its Thai and English websites (www.ddc.moph.go.th). Any persons who may have specific medical or public heath concerns about this event are invited to report them or make queries using email address: askcmevent@gmail.com