A Quebec coroner is challenging the autopsy findings of Thai officials that ruled two Quebec sisters found dead in their hotel room in June were accidentally poisoned.

Coroner Renée Roussel told Radio-Canada the concentration of the chemical DEET in the sisters' systems wasn't enough to be fatal.

That contradicts the conclusion of Thai authorities, who performed post-mortems on the bodies of Noémi Bélanger, 25, and Audrey Bélanger, 20, shortly after the sisters were found on June 15 by hotel staff.

A pathologist determined the women likely ingested DEET, a principal ingredient in bug repellant, in a euphoria-inducing cocktail that is popular among youth in Thailand.

The sisters from Pohénégamook, Que., had just arrived on Thailand's Phi Phi Island and were last seen partying with two Brazilian friends in the early morning of June 13.

Investigators said there were no signs of foul play in their hotel room, but there was evidence that the women may have suffered some kind of toxic reaction.

DEET levels not fatal

Dr. René Blais of Quebec's poison control centre said the DEET concentration reported by the Thai pathologist doesn't correspond to a concentration that would be toxic, "let alone a concentration that would be fatal."

It's still unclear what caused their deaths if it wasn't DEET poisoning.

Secondary autopsies were conducted in Montreal, but the results haven't been released.

Thai investigators haven't closed the case. They submitted their investigation report to the Canadian Embassy in Thailand without making the findings public.

Other mysterious deaths

In the last three years, a dozen vacationers have died under suspicious circumstances in tourist areas of Thailand and Vietnam.

In 2009, two young tourists, one from the United States and the other from Norway, who were staying at a guest house near the hotel where the Bélanger sisters were found, also died under mysterious circumstances.

Their deaths remain unsolved, but there was speculation the women had been poisoned.