Six Vietnamese people died of poisoning at a Bangkok hotel on Tuesday evening.
Lumpini police were alerted to the incident at the Grand Hyatt Erawan Hotel in Pathum Wan district around 5.30pm.
Officers found three men and three women dead inside a room, with no signs of a struggle. Their luggage was found near the front door.
Police were examining closed-circuit television (CCTV) footage and questioning witnesses to gather possible clues.
Investigators identified the deceased as two Vietnamese individuals with US nationality and four Vietnamese nationals.
No bruises were found on their bodies, and it was later confirmed that they died of poisoning.
However, it is still unclear whether they took their own lives or were poisoned by someone else.
Food and drinks discovered inside a Bangkok hotel room where six people were found dead on Tuesday. (Photo supplied)
Pol Maj Gen Theeradet Thumsuthee, investigative commander of the Metropolitan Police Bureau, confirmed that it was likely the six people died of poisoning and that they appeared to have consumed coffee and tea in the room.
Detectives had yet to narrow down possible motives for the deaths, he said.
Local media reported that the six foreigners may have died of cyanide poisoning.
Bangkok police commissioner Pol Lt Gen Thiti Saengsawang said that seven people checked in for five rooms on the fifth and seventh floors over the weekend. Four rooms on the seventh floor were scheduled for check-out on Monday afternoon, and their bags were packed.
"We need to find out the motives," Pol Lt Gen Thiti said. "What we can prove now is that they are not dead from suicide but from the killing of others."
The food served to their room was left uneaten, while cups were empty and drinks appeared to be consumed, he said.
Only one victim had a facial wound, and police assumed the person fell onto a hard object, he said.
Two victims, a man and a woman, seemed to have tried to reach the room's door, he added.
National police chief Pol Gen Torsak Sukvimol said the room was locked from the inside when the officers arrived at the scene.
Two of the dead were identified as Chong Sherine, 56, and Dang Hung Van, 55. Both had American citizenship.
The four others were Nguyen Thi Phuong Lan, 47, Pham Hong Thanh, 49, Tran Dinh Phu, 37, and Nguyen Thi Phuong, 46.
Prime Minister Srettha Thavisin went to the scene Tuesday night. He said police assumed the people had been dead for about 24 hours before being discovered and that authorities were looking for another Vietnamese citizen.
"We need to conduct an autopsy to see if they had ingested anything," the prime minister said, dismissing rumours linking the deaths to theft or a shooting.
The incident was unexpected, but it should not have any negative impact on tourism, he said.
The prime minister has ordered officials to take urgent steps to contain panic among tourists and the public, according to government spokesman Chai Wacharonke.
Police gather outside a Bangkok hotel room where six people were found dead on Tuesday. (Photo: Royal Thai Police)