PHUKET: The mysterious deaths of Canadian sisters Audrey and Noemi Belanger on the holiday island of Phi Phi were caused by insecticide, police said yesterday.

No more details are available at this stage, a Thai officer said, although it has been 12 weeks since Audrey, 20, and Noemi, 26, were found dead in their room at a solid, respectable resort on June 15.

Since then, as autopsies on the women were carried out first in Bangkok and then in Canada, secrecy has enveloped the double fatality.

Beyond saying that insecticide has been found in the bodies of the women, police in Krabi, the mainland province closest to Phi Phi, have little to add.

It could be three months before official results are revealed if ever, Krabi Town Deputy Superintendent Colonel Jongrak Pimtong told Phuketwan yesterday.

All the signs in the room at the Palm Residence Phi Phi were that the sisters died a horrible death from some kind of toxic substance. Much speculation has followed about everything from an allergic reaction to a serial killer.

The tragedy of the Canadian sisters mirrors the equally mysterious deaths on Phi Phi of Jill St Onge, 27, and Norwegian Julie Bergheim, 22, in May, 2009.

Comparisons have also been made with a series of unexplained deaths - coincidentally or not, mostly involving young women - at and near the now-demolished Downtown Inn in the Thai city of Chiang Mai last year.

One big difference is that the father of the most prominent of the Chiang Mai victims, 23-year-old new Zealand tourist Sarah Carter, conducted a very public campaign to find out what killed his daughter.

The Thai authorities were forced to instigate a full and proper probe, with regular updates on their findings.

Although there was no formal conclusion, the likelihood is that pesticide killed Sarah Carter and the others in Chiang Mai.

Pesicide in Chinag Mai, insecticide on Phi Phi . . . Whatever the killer, it was no ordinary bug spray.

Could someone have doctored their drinks? Others with Ms St John and Ms Bergheim also fell ill and survived, as did the companions of Ms Carter.

Phi Phi is renowned as a rites-of-passage destination for 20-somethings, and it transforms from a haven for day-trippers in the sunshine to a less beguiling party island after dark.

Alcohol is just one the many ingedients that Phi Phi's party people mix in their ''buckets.''

Each bucket is a concoction of all kinds of juices and substances that are mixed into containers of various sizes and usually sucked through straws all night long.

There is no evidence, however, that Ms Bergheim and Ms St Onge, who fell sick in adjacent rooms at the same Phi Phi guesthouse, shared a meal or a drink.

Ms St Onge was cremated but Ms Bergheim's body was returned to Norway where a second autopsy that extended over a period of months failed to produce a reason for her death.

It is believed that the post-autopsy tests for the Belangers have concluded in Bangkok but are continuing in Canada.

Public Health Department officials in Thailand say they check for contaminants in the Phi Phi ''buckets'' once a month but have found none.

That could mean an inordinately large dose of pesticide or insecticide was inhaled, swallowed or absorbed by the Belanger sisters some other way.

Unlike Sarah Carter's father, the parents of the Belangers, despite the shocking double loss, are maintaining a consistent silence, probably on the advice of Canadian authorities.

When asked a series of questions this week, a government spokesperson in Canada responded: ''Our thoughts continue to be with the family and friends of the Canadian citizens who passed away in Thailand.

''Canadian consular officials in Bangkok and in Canada are providing consular assistance to the family.

''Our officials are in contact with authorities both in Thailand and Canada and will continue to liaise closely with them.

''However, out of respect for the family and to protect the privacy of the individuals concerned, further details on this case cannot be released.''

Despite the calm in Canada, fresh alarm surfaced earlier this month with reports of the deaths of two young women in Vietnam, an American and a Canadian of Vietnamese extraction.

There is a sense of deja vu about the reporting of the riddle in thespec.com, the online news site of a respectable Canadian publisher.

''[Cathy] Huynh, 26, . . . and Kari Bowerman, 27, of Lake Geneva, Wisconsin, died suddenly and mysteriously in Vietnam while on vacation together from their English teaching jobs in South Korea.

''Both were healthy young women before travelling to Vietnam, according to their families. News reports in Vietnam said they had been vomiting and sought help at a local hospital, where they died - Bowerman first on July 30 and then Huynh on August 2.''

The report adds: ''In Texas, Bowerman's sister Jenny Jaques is not only in shock and grieving, as is the Huynh family in Hamilton, but she is also angry and frustrated at not being able to get information about what happened.''

Little has been reported about the case since, and the same can be said of the Belanger sisters mystery.

Despite the deaths and the continuing mysteries, disquiet does not appear to have affected tourism to Phi Phi.

Crowded ferries still come and go each day to the scenic island from Phuket, about two hours' sailing away. Many of the passengers are young women.