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Members of the media film a forensic police layout of the room where six people were found dead the day before allegedly due to poisoning, during a press conference at Lumpini Police Station in Bangkok on July 17, 2024. Image Credit: AFP

The deaths of six foreigners at a luxury Bangkok hotel are suspected to have been caused by cyanide poisoning, Thai police said on Wednesday, as the US Federal Bureau of Investigation joined local authorities in probing the mysterious killings.

Preliminary forensic evidence pointed to traces of cyanide in the blood samples of one of the victims and in a thermo flask and cups found in the hotel room, Trairong Phiwpan, commander of the Office of Police Forensic Science, told a press briefing on Wednesday.

The police ruled out the involvement of a seventh person as suspected earlier, according to Nopasin Poolsawat, deputy commissioner of the Metropolitan Police. He said the deaths were not being treated as a transnational crime but a personal one.

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Thai police suspect one possible motive for the poisoning was a fallout of a business deal gone awry as four of the victims had financial dealings worth 10 million baht ($278,303) among them, Nopasin said.

Police are trying to unravel the mystery behind the deaths, less than 24 hours after a hotel maid found the six people, lying dead in their room. There was no signs of any struggle and no significant bruises or wounds on the dead bodies. Room service food had been left untouched, though there were six tea cups containing "residues" at the bottom.

Earlier on Wednesday, FBI agents joined Thai police at the Grand Hyatt Erawan Bangkok hotel because two of the guests found dead at the hotel were US citizens, according to Prime Minister Srettha Thavisin.

Five hotel rooms were booked for seven guests, but only five people had checked in. The five were found dead by a cleaner in a fifth-floor suite, along with another person, leaving one as a suspect, Thiti Saengsawang, chief of Bangkok's Metropolitan Police Bureau, said in a briefing on Tuesday.

All six victims were Vietnamese, with two of them holding US citizenship. The deceased comprised three women and three men aged from 37 to 56 years old, authorities said.

Tourist hotspot

Shares of Erawan Group Pcl, the operator of Grand Hyatt, fell as much as 5.2 per cent in Bangkok trading to the lowest since September 2022.

The hotel is located near the downtown Ratchaprasong intersection, an area popular with foreign tourists. But the deaths will not affect the country's tourism industry, Srettha said, adding that an event at the hotel to be attended by the visiting Russian energy minister will proceed as scheduled.

Tourism is one of Thailand's key industries, accounting for about 20 per cent of total jobs and making up roughly 12 per cent of the nation's $500 billion economy. Srettha's administration has set a goal of attracting 80 million tourists by 2027.

Foreign arrivals to Thailand this year through July 14 rose about 35 per cent from the same period in 2023, to 18.9 million, generating revenue of 891 billion baht ($25 billion), the Tourism and Sports Ministry said earlier Tuesday. China, Malaysia and India were the biggest sources of tourists so far in 2024.