A potentially lethal toxin which causes symptoms similar to those which New Zealander Sarah Carter had before she died has been found in her Chiang Mai hotel room.
Thai authorities said the chemical compound was thought to be an insecticide for bed bugs.
In February, Ms Carter died in hospital from a heart inflammation two days after she and two friends became sick while staying at the Downtown Inn.
Seven people have died in similar circumstances to Ms Carter - including a Canadian tourist, after using the facilities of the Downtown Inn, an elderly British couple and a Thai tourist guide.
A United Nations scientist told 60 Minutes there was a strong likelihood that Miss Carter's death was a result of excessive exposure to the substance, which has been banned for indoor use in other countries.
Dr Ron McDowall said the toxin causes symptoms identical to those suffered by the tourists who have died.
One of the three women, Emma Langlands, last night spoke for the first time of the events leading up to their becoming sick.
The three friends had arrived in Chiang Mai in the afternoon, settled at the Downtown Inn and booked a tour for the next day before heading out that evening for dinner.
Speaking about the hotel the group stayed at, Ms Langlands said there was "nothing that really stood out" which might indicate something was wrong with it. "The room seemed really clean, there were no strange smells - we did [however] see a sign that said the hotel was being sprayed with insecticides."
It is understood the company which sprayed the Downtown Inn is now under investigation. A day before public officials were due to come and inspect the area, the whole of the fifth floor - the floor in which the girls stayed - was stripped.
Hidden camera footage showed cleaners vacuuming, stripping carpets and wiping down surfaces.
A 60 Minutes reporter is shown sneaking into the room in which the girls stayed and taking samples from surfaces in the room.
The samples, which were brought back to New Zealand and tested, showed traces of a poison called pyrophus, used in insecticides and which is banned in some countries.
A Kiwi scientific expert told the programme he believed there may have been a bedbug infestation at the hotel, and coupled with an overuse of insecticide, resulted in guests becoming sick.