Canadian backpacker Cathy Huynh had only just arrived in the coastal Vietnamese city of Nha Trang when she and her travel companion Kari Bowerman went to a hospital with breathing difficulties.

Three hours later, Ms. Bowerman died of respiratory failure. Two days after that, Ms. Huynh was also dead after going into shock.

Although Vietnamese authorities said this week they still did not know what caused the sudden deaths, the women’s final hours show all the signs of being caused by chlorpyrifos, a potent anti-bedbug poison linked to tourist deaths across Southeast Asia.