BANGKOK >> Cyanide poisoning caused the deaths of six foreigners whose bodies were found in a room in a plush Bangkok hotel, Thai authorities said on Wednesday, with the suspected killer among the dead.

Traces of the rapid-acting, deadly chemical were found during autopsies of the bodies and on drinking glasses and a teapot in the room at the luxury Grand Hyatt Erawan hotel where the dead were discovered late on Tuesday, according to police and a hospital.

Interviews with relatives of the three women and three men who died revealed there had been a dispute over debt related to an investment, according to police, who said they were investigating how the cyanide was obtained.

The six were all of Vietnamese ethnicity, two of those U.S. nationals. Police said the U.S. Federal Bureau of Investigation had assisted with the investigation.

“We can assume that the six died from cyanide,” Chulalongkorn Hospital’s Kornkiat Vongpaisarnsin told reporters, adding that the results of further tests would be available on Friday.

The U.S. State Department said it was monitoring the situation and local authorities were responsible for the investigation.

Vietnam’s foreign ministry confirmed four of the dead were Vietnamese nationals and its embassy in Thailand was coordinating closely with authorities.

“We hope that the victims’ families soon overcome this great loss,” foreign ministry spokesperson Pham Thu Hang said.

The Grand Hyatt Erawan, operated by Erawan Group, 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.

The government had been eager to quickly get to the bottom of what happened, concerned bad publicity could impact a sector that is expecting 35 million foreign visitors this year and tens of billions of dollars in spending.

Trirong Phiwpan, commander of the Thai police evidence office, said the investigation indicated drinks in the hotel room were spiked with cyanide by one of those who died.

“After staff brought tea cups and two hot water bottles, milk and tea pots… one of the six introduced cyanide.”