updated 8/16/2011 4:53:06 AM ET 2011-08-16T08:53:06

An investigation into the mysterious deaths of a U.S. woman and four other tourists in hotels in northern Thailand has suggested a link to toxic chemical exposure, but failed to determine exactly what killed them, according to a government website Tuesday.

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The victims — from New Zealand, France, and Britain as well as the U.S. — were staying at three different hotels in the popular northern Thai city of Chiang Mai.

A Thai tour guide also died, and three other tourists fell ill but recovered.

No one has been declared responsible in the spate of deaths, which came in January and February.

"The specific agents that caused the deaths and illnesses in these events cannot be identified, and it can not be determined exactly how people were exposed to them," the Department of Disease Control said on its website Tuesday.

The results were most revealing concerning a 23-year-old New Zealand woman who died Feb. 6, her two females companions — both also 23 — who got sick but recovered, and a 47-year-old Thai woman who died February 3.

The four are "most likely to have the same cause of illness, probably exposure to some toxic chemical, pesticides or gas," the report states.

The deaths of a British couple in their 70s found in their hotel room Feb. 19 were "possibly related" to the those deaths "as they occurred in the same hotel," the report states.

'Chemical or biotoxin'
Those deaths happened at the Downtown Inn in Chiang Mai. The hotel owner declined to comment Tuesday.

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In the case of the 33-year-old American woman who died January 11 "the suspected agent that caused her death is likely to be chemical or biotoxin in nature and it is probable that it might have been a pesticide," the report states.

She was staying in a different hotel in the same area and a 29-year-old Canadian woman traveling with her fell ill but recovered.

The final death, a 25-year-old French woman who died January 19, the report starts "developed fever and other symptoms on 16 January before arriving in Chiang Mai" so her case is probably not related to the others.

The website added authorities are taking measures to reduce the risks to visitors, but it did not give details.

Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.


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