Bangkok, Thailand - Six foreign nationals were found dead in a luxury Bangkok hotel after presumably ingesting cyanide from teacups, in a mystery that shocked Thailand.

The bodies of six people of Vietnamese origin – two of whom held US citizenship – were discovered in a suite at the Grand Hyatt Erawan Hotel in the tourist center of Bangkok on Tuesday afternoon.

Photos released by police of the bloodless crime scene showed bodies scattered around the room among plates of untouched Thai food, and two thermos flasks and cups.

Initial examinations revealed the presence of cyanide in six cups, according to police.

"The mouths and nails on all the bodies turned purple, showing a lack of air could be a joint reason of the cause of the death," forensic doctor Kornkiat Vongpaisarnsin told a press conference at the Chulalongkorn University.

"We presume they all died from cyanide which causes a lack of air in some organs," he added.

Earlier, Thai police said one of the six people found dead likely also carried out the poisonings, thought to be related to debts worth millions of baht.

"We are convinced that one of the six people found dead committed this crime," said Noppasil Poonsawas, a deputy commander of Bangkok police.

The mysterious circumstances surrounding the deaths of the three men and three women, aged between 37 and 56, have fuelled wild rumors, with several local media outlets initially reporting a mass shooting incident.

Investigators also revealed that no movement had been observed around from Monday afternoon, suggesting the deaths could have occurred the day before the bodies were discovered.

Speculation a seventh person may have been part of the group was also dispelled by the police.

Tran Dinh Dung, the father of one of the victims, said his 37-year-old son was due to return to Vietnam last Sunday.

"I kept calling him but couldn't get through so I was very worried, but I didn't expect him to die in Thailand," Dung said in an interview with Vietnam's Thanh Nien newspaper.

"Phu's mother has fainted countless times, she could not bear this shock," he said.

Prime Minister Srettha Thavisin said Wednesday the crime was "a private matter", unrelated to national security, and should not affect the lucrative tourist sector.