Two tourists have died in Vietnam and they are the latest in a series of tourists that in recent years have died in Southeast Asia, reports CNN.

The tourists were Cathy Huynh, age 26, and Kari Bowerman, age 27. The two were friends working in South Korea as English teachers and they went on vacation to see sites in Vietnam. However, something took hold of the women giving them severe dehydration and vomiting. Within two days of arriving at a local hospital the women had died.

The official cause of death is unknown but investigators with the World Health Organization suspect poisoning due to unknown origins. Over the past three years, tourists have mysteriously died across Southeast Asia but the cause hasn’t been determined.

One investigation in Thailand, found levels of chlorpyrifos, a toxin used to kill bedbugs.

If those levels are elevated, they can cause the symptoms found in the these victims. However, at this time, no cause has been determined from these deaths which all similar symptoms.