Six Vietnamese people were found dead 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 clues.
Investigators identified the dead 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.
Local media reported that the six foreigners may have died of cyanide poisoning.
Police had yet to narrow down possible motives for the deaths.
Prime Minister Srettha Thavisin went to the scene Tuesday night. He said police thought the people had been dead for about 24 hours before being found and police were looking for another Vietnamese citizen.
The prime minister said there were no signs of robbery or assault. The incident was unexpected but it should not have any negative impact on tourism, he said.
