THE MYSTERY woman in Phi Phi's Laleena Guesthouse double-death riddle has spoken publicly for the first time. And she agrees that the killer was . . . gas.

This has always been the view of American Ryan Kells, whose fiancee Jill St Onge, 27, was one of two young women tourists who fell ill at Laleena and later died.

The other woman killed without any clear reason was Norwegian Julie Bergheim, 22.

Her travelling companion, Karina Refseth, also fell sick from the same cause, as did Ryan Kells. But Refseth and Kells survived, Refseth narrowly after emergency care.

Kells has always maintained that he sniffled a chemical substance when booking in to a Laleena room with St Onge on that Saturday in May, 2009.

St Onge, Bergheim and their companions occupied Rooms 4 and 5 on the ground floor at the guesthouse, but the couples probably never met before the two women died in a similar, horrible fashion.

Now, after seven months of silence, Refseth, the mystery witness, has finally spoken out. She too believes the killer was gas.

The problem is that two autopsies in Thailand and a third extremely unusual autopsy in Norway have failed to produce an official cause of death.

How could two young tourists, simply through fate staying in adjoining rooms in a guesthouse on one of the world's best-known tropical holiday islands, suddenly fall ill and die?

There is no indication that they ever met or shared any similar experiences on Phi Phi, except for being guests at Laleena.

Surely the days have passed when people can perish without a cause being determined? This is, after all, 2010, the 21st century.

In recent days, Phuketwan has talked to virtually all the key people connected to the deaths of the two young female tourists on Phi Phi.

The Norwegian pair had been travelling home after spending a year studying in Australia and had been on Phi Phi for a few days, staying at Laleena Guesthouse, when Ryan Kells and Jill St Onge checked in next door in Room 4 on May 2.

Before the weekend was over, two young women were dead, and the third was dangerously ill.

Soon afterwards, a barely recovered Ryan Kells was being carried off the island with the body of his fiancee in a bag in the bottom of the speedboat.

It is probably an understatement to say that he remains ill at ease with virtually every aspect of the investigation by Thai police into the double fatality.

Kells had been on a once-in-a-lifetime trip with St Onge and proposed to her as they journeyed through Asia. They moved to Laleena that day to have some air-conditioning for a change.

This is what he told us via email last week:

''The only thing I can say is that there is no way it was a bacteria or food or drink poison. Jill had NO ALCOHOL that night. and food poisoning can not affect a person so rapidly.

''Nor can a bacteria. And a bacteria would be easily seen in an autopsy. So I would really like you not to say that those had any part of it.''

St Onge's body was examined by Thai forensic pathologists in Bangkok, tissue and blood samples were taken, then she was cremated. Her ashes went back to the US with Kells.

By contrast, Bergheim's body went back to Norway where family and officials in her home county, aghast at such an appalling catastrophe, contrived to have a second autopsy carried out by some of Norway's top forensic pathologists.

The Thai autopsy report was condemned by St Onge's relatives for its brevity. And it must be said that the whole Thai investigation into the case proved to be singularly inadequate.

When a tourist dies in a popular holiday destination in Thailand, there is still a tendency to cover up, even to conceal the event entirely if possible, for the sake of the country's tourism industry.

While this attitude inevitably has consequences in an age of total transparency, Thailand has yet to prove it is unequivocally committed to the safety and security of the visitors who provide the country with a large slice of annual revenue.

Time and again, corruption or the fear of negative consequences become factors that obscure justice or prevent an honest recognition of the causes of man-made disasters.

Just as Kells has always insisted that he thought the demise of his bride-to-be was triggered by some kind of chemical, Laleena Guesthouse owner Rat Chuped has consistently maintained that her premises had nothing to do with the deaths.

Laleena's 10 rooms have been occupied since Christmas, says Khun Rat, who adds that guests occasionally ask about the deaths.

She says that she always keeps the guesthouse scrupulously clean and long ago reported that she slept in one of the rooms in question soon after the deaths.

When asked this week what caused the deaths, she said: ''No idea.''

This was also the conclusion of a belated Thai university laboratory check on various cleaners and chemicals found and sampled from the guesthouse.

The Thai police involved in the initial inquiry have moved on. Their successors expressed little interest in its outcome.

But in Norway and the US, people still want to know.

For months in Norway, senior forensic pathologists pursued tests resulting from their unusual second autopsy. While no offence was intended to Thai authorities, the message was plain.

Just before Christmas, a media release came from the office of public prosecutor Sjak R. Haaheim. He reported:

''Miss Bergheim died at a Thai hospital May 4 2009, after having been acutely ill during her stay there. Thai police are investigating the case, and have submitted case documents as a valuable input into the Norwegian inquiry.

''This hasn't been a criminal investigation. We have performed certain inquiries in Norway, to seek answers as to why Miss Bergheim unexpectedly died. The results of our inquiries will be made available to Thai authorities through ordinary diplomatic channels.

''Thai forensic experts performed an autopsy on Miss Bergheim, related to the criminal investigation.

''When her body arrived to Norway in June, the Norwegian Prosecution Service decided to perform another autopsy, hoping that this could bring forward more answers to the cause of death.

''The autopsy was performed at RMI, the National Forensic Institute, in Oslo on June 16, 2009. Several samples were obtained and later analysed. Unfortunately, the cause of death cannot be determined with certainty.

''However, RMI concludes there are several possible causes of death, including poisoning from bacterias, poisonous food or drinks, or through chemical agents that may have been distributed through the local environment.''

To their credit, the Norwegians were not prepared to accept that one of their much-loved daughters could die in this fashion, without diagnosis. Sadly, their efforts appear to have failed.

Then there are the survivors. Finally, after not commenting publicly for months, Refseth spoke out: ''We reacted to the smell in the room when we arrived. The chemical smell, but thought no more about it,'' she is quoted as saying.

It was three days later when Kells and St Onge checked in, and Kells also noted the chemical smell.

''It was a terrible experience, and this is very difficult, '' Refseth says. ''I think they should find out what made us sick, what led to Julie's death.

''It is very difficult not knowing, not having been given proper answers to what really happened.''

Kells, the other survivor, is equally dissatisfied. ''My best friend died right in front of me on a vacation of a lifetime. I will not be told 'no one knows' and be ok with that.

''The thing that can be achieved is, FIND OUT WHAT KILLED JULIE AND JILL.That is all I care about, and I promise to find answers if no-one else will.''

From Norway, public prosecutor Sjak R. Haaheim was more sanguine: ''Sometimes in life, and in death, there is no possibility of answers,'' he concluded.