The rapid-acting, deadly chemical was found on drinking glasses and a teapot in the room at the luxury Grand Hyatt Erawan hotel, and interviews with relatives of the dead revealed there had been a dispute over debt related to an investment, police said.
The six were all of Vietnamese ethnicity, two of those U.S. nationals, and were found dead late on Tuesday. The U.S. Federal Bureau of Investigation had assisted police with the investigation, police said.
"We found cyanide in the teacups, all six cups we found cyanide," Trirong Phiwpan, Commander of the Thai police evidence office, told a press conference.
"After staff brought tea cups and two hot water bottles, milk and tea pots... one of the six introduced cyanide." The results of an autopsy were expected within the next day, police said.
Vietnam's government said its embassy in Bangkok was closely coordinating with Thai authorities on the case, while the U.S. State Department said it was monitoring the situation and local authorities were responsible for the investigation.
The Grand Hyatt Erawan, operated by Erawan Group ERW.BK, has over 350 rooms and is located in a popular tourist district known for luxury shopping and restaurants.
News of the deaths, initially reported by some Thai media as a shooting, could be a setback for Thailand as it bets heavily on its vital tourism sector reviving an economy that has struggled since the pandemic.
Thailand is expecting 35 million foreign arrivals this year, up from 28 million last year who spent 1.2 trillion baht ($33.71 billion).
Prime Minister Srettha Thavisin on Tuesday urged a swift probe into the issue to limit the impact on Thailand's travel sector.
