Six people have been found dead at a luxury hotel in central Bangkok, in what authorities are beginning to look into as a possible case of poisoning. The bodies of three women and three men were found inside a hotel suite on the fifth floor of the Grand Hyatt Erawan Hotel by a hotel worker late on Tuesday afternoon.

According to a statement from Thai Prime Minister Srettha Thavisin at a press conference following the discovery of the bodies, all six individuals were Vietnamese, and two had American citizenship. It is currently assumed by authorities and media forces alike that the six had been dead for at least twenty-four hours before their bodies were discovered. The room on the fifth floor had been locked from the inside, but the staff member could enter through a separate door.

The cause of death is not yet known. However, Srettha reiterated once more that there were no signs of robbery or an attack, going on to say that the cause of death was “presumably something related to consumption, which needs to be investigated.”

Police shared images of hotel food in the suite, which was still sealed in clingfilm and appeared entirely untouched. However, authorities said six cups seemed to have been used inside the suite.

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Metropolitan police bureau commissioner Thiti Saengsawang noted, “The food was untouched but all six cups were used. We will check all of it. We could not find anything else around, even on the floor, but we found some kind of powder in the bottom of a cup.”

“We need to find out the motives,” he said, adding that the deaths were not the result of suicide but of a “killing.”

Srettha said he had met the Vietnamese ambassador to discuss the matter and had ordered a swift investigation into the case. The deceased had been booked to stay at the hotel as a group of seven people, but only five officially checked in, adding to the confusion since six bodies were found. Police said they were now searching for the seventh individual.

“We are tracing every step since they got off the plane,” Thiti said. Part of the group was staying on the fifth floor and was due to check out on Tuesday, while others were allegedly waiting on the seventh floor and had been due to check out on the previous Monday but had failed to do so. Images taken inside the hotel showed their luggage had been packed up.

The Grand Hyatt Erawan is a five-star hotel in Bangkok’s commercial and diplomatic district.

“The prime minister has ordered all agencies to urgently take action to avoid impact on tourism,” the Thai government said in a statement. The US State Department was “closely monitoring the situation, and [we] stand ready to provide consular assistance,” a spokesperson said.

The incident comes at an unfortunate time for Thailand’s tourism industry, which has been badly affected by the pandemic in recent years.