Two Americans were among six people confirmed dead at a luxury hotel in Bangkok.
Four Vietnamese nationals and two Vietnamese Americans were found dead with no sign of a struggle in the Grand Hyatt Erawan Hotel on Tuesday.
A seventh guest was included in the original booking but has yet to be located.
The victims were found foaming at the mouth and police were concerned they may have been poisoned.
They were checking for any substances that could have been added to the group's drinking glasses.
Thailand's Prime Minister, Srettha Thavisin, addressed a crowd of journalists and confirmed that the six victims were found in a room at the Grand Hyatt.
A police officer said the victims were three men and three women.
Housekeeping happened upon their bodies after they failed to check out.
According to the officer, "suspicious substances" were found at the bottoms of glasses found in the hotel room, and authorities are focusing on a "toxic substance."
Channel News Asia posted two photos of the crime scene.