The same pesticide that killed two children in Fort McMurray, Alta. last month has been identified by a coroner as the probable culprit in the death of two Quebec sisters found unconscious in their Thailand hotel room in 2012.

In a report Monday, coroner Renée Roussel said phosphine, the gas released by aluminum phosphide, is the most likely poison to have killed Audrey Bélanger, 20, and her sister Noémi, 25, while they were staying at a resort on Phi Phi Islands.

The two sisters from Pohénégamook, Que., were part of a cluster of travellers who died in unexplained circumstances in Southeast Asia.

Story continues below advertisement

Dr. Roussel said she was unable to formally confirm phosphine's role because it left no traces in the remains of the sisters. Lengthy toxicology tests failed to find signs of nearly 800 possible harmful chemicals, Dr. Roussel told reporters.

The only substances the tests detected were proguanil, a medication to prevent malaria, and diethyltoluamide, the insect repellent widely known as DEET.

They found no mitragynine, the active ingredient in the Asian plant kratom. This dispelled speculation that the sisters died from ingesting "bucket drinks," popular cocktails on Phi Phi that mix cola and stimulants such as ground-up kratom leaves and DEET.

The Thai autopsy initially blamed DEET as the cause of death. However, the DEET level in the sisters' blood – 15.3 micrograms per litre – was too low to be toxic and remained consistent with its use when applied as a skin cream, Dr. Roussel said.

The investigation then looked at substances that could kill quickly but would be so volatile they wouldn't leave traces. Dr. Roussel said this pointed at aluminum phosphide. Sold in tablets, pellets or small sachets of powder, aluminum phosphide releases a toxic gas, phosphine, when it comes into contact with moisture in the air.

Last month, a two-year-old boy, Zia Hassan, and his eight-month-old sister, Zara, died after their parents used phosphine brought into Canada from Pakistan to fight bedbugs in their Fort McMurray apartment.

A highly toxic but cheap farming product, aluminum phosphide is widely used in developing countries and has become one of the most common causes of poisoning among agricultural pesticides, according to a 2011 paper of the Journal of Emergencies, Trauma and Shock. In Canada, it is used to fumigate grain elevators and kill pests on shipping vessels.

Story continues below advertisement

Following the two children's death, Health Canada said phosphine can only be sold to licensed professionals and is not approved for use against bedbugs.

Similarly, in Thailand, it is illegal to use phosphine indoors, but Dr. Roussel said "we believe it was likely used anyway." Her report noted that the region's hotelkeepers struggle to control insects, especially bedbugs, in hotel rooms.

Brain-cell samples from the sisters showed signs of damage caused by an acute lack of oxygen, a symptom consistent with exposure to aluminum phosphide, the coroner said. Her report noted that, since 2009, about 20 foreign travellers have died in similar fashion in Southeast Asia.

Two women, Julie Bergheim of Norway and Jill St. Onge of the United States, died in 2009 at another hotel in Phi Phi, with symptoms similar to the Bélanger sisters. The report said FBI agents consulted Canadian officials and came to similar conclusions pointing at phosphine.

The Bélanger sisters were well-liked students at Laval University who hailed from a small town downriver from Quebec City. They were concluding a month-long trip to Asia when they checked into the Phi Phi Palms Residence on June 12, 2015. Their bodies were discovered June 15.