Thai investigators are looking for two Portuguese men, who were seen on closed-circuit cameras escorting Noemi and Audrey Belanger back to their hotel room, according to the Bangkok Post.
They may have been the last people to see the Quebec sisters alive.
Noemi, 26, and Audrey, 20, were found dead at the Phi Phi Palms Residence Hotel on the popular resort island on June 15.
"Police determined they were dead for about 24 hours prior to that and only found a lot of vomit in the room," Lt. Col. Jongrak Pimthong told The Associated Press.
"There were neither signs of fighting, nor robbery, but we found many kinds of over-the-counter-drugs, including ibuprofen, which can cause serious effects on the stomach."
Initially, local authorities suspected the girls had been poisoned, as findings seemed to suggest a 'toxic substance'.
The Post is reporting that the CCTV footage captures a man taking the sisters back to their room, between 1:10 and 1:14 am. The newspaper identifies the man as 30-year-old Portuguese national Luciano Tinto, who was staying at the same hotel as the Belangers.
Citing a source close to the investigation, The Post reports the sisters were spotted at various night spots on Phi Phi island alongside Tinto and another unidentified Portuguese national.
The Belangers grew up in Pohenegamook near the Maine border in Eastern Quebec, where they worked in the family shop.
Town mayor Louise Labonte described them to the Toronto Star as adventurous and full of promise.
“We lost two brilliant young women,” the mayor told the newspaper Sunday. “It’s of course very difficult for the family and everyone here.”
Relatives of the sisters arrived in Thailand earlier this week. The women's bodies are reportedly back in Canada, according to CBC News.
In the same report, their uncle, Eric Belanger, told the CBC he wasn't certain when the family will learn the results of toxicology tests conducted in Thailand.