By South-East Asia correspondent Lauren Day in Bangkok, ABC
All of these tea cups had traces of cyanide. Photo: Royal Thai Police / AFP
When a waiter delivered food and tea to Sherine Chong's hotel room, he could see she was stressed.
The walls of the luxury room in Bangkok's Grand Hyatt Erawan were closing in on the 56-year-old, who was under pressure, in huge debt and facing legal action over a business dispute.
She had been lent hundreds of thousands of dollars to invest in a project to build a hospital in Japan, but there had been no progress and the investors wanted their money back.
With the parties scheduled to meet in court in two weeks, she had invited them to Bangkok for out-of-court negotiations.
On Monday, the group gathered in Chong's room at the swanky hotel in central Bangkok for lunch and tea.
When the waiter offered to brew the tea, Chong refused and said she would prepare it herself.
Just over 24 hours later, all six people were found dead in that hotel room, their lips and fingernails black - the telltale signs of cyanide poisoning.
Bowls of tom yum soup and fried rice were untouched, but all six tea cups had traces of the deadly poison.
Police said they believed one of the six had killed the others before taking their own life, but did not name a suspect.
However, they added Chong was the only person in her room when the food and tea were delivered.
An investment gone bad
Police said two members of the group - married couple Nguyen Thi Phong and Pham Thanh Hong - had given Chong more than $400,000 for the hospital project.
According to the BBC, they also suspected her personal make-up artist for the trip, 37-year-old Tran Dinh Phu, had been duped into putting in some of his own money.
When the hospital project stalled and the couple took legal action, Chong asked to meet.
The group had initially planned to go to Japan, but some members had visa issues so they decided on Bangkok instead.
Some members tacked on tours and had other travel planned for when business was out of the way.
After the five other members of the group checked out at noon on Monday, Chong invited them to her room for a final round of talks over room-service food and tea.
CCTV showed all six members of the group gathering outside the room and then entering - the last time they were seen alive.
The following day, when a maid went in to check the room after Chong failed to check out, she found the three men and three women dead on the floor.
Four were found in the living room, and two in the bedroom.
One of the men had been apparently trying to reach the door but collapsed before he was able to.
The dead - ranging in age from 37 to 56 - were all of Vietnamese descent, with two of them United States citizens.
Thai Police have been coordinating with the embassies of Vietnam and the United States to solve the case, and officials from the Federal Bureau of Investigation have travelled to Bangkok to assist.
Vietnam's foreign ministry said its embassy in Thailand was coordinating closely with local authorities.
"We hope that the victims' families soon overcome this great loss," spokesman Pham Thu Hang said in a statement.
The six met at the Grand Hyatt Erawan. Photo: LILLIAN SUWANRUMPHA/AFP
Questions over how killer accessed cyanide
Autopsy reports from Bangkok's Chulalongkorn Hospital quickly confirmed what police had already suspected - all six bodies contained traces of cyanide.
Initial results found signs of asphyxiation and unusual congestion and colouring of the blood - typical of cyanide victims.
Professor Winai Wananukul from the Ramathibodi Poison Center said the fast-acting poison cuts off the supply of oxygen.
"It has effect of making cells in the body unable to use oxygen, so it has the same effect as when the body is deprived of oxygen," he told the ABC.
"Cyanide acts very fast in a few minutes of ingesting by eating or inhaling."
He said it was unclear how the cyanide had come into the alleged killer's possession and whether it was procured in Thailand or brought into the country from Vietnam.
"It is already a controlled substance, but how much we can enforce that is a different issue," he said.
Thailand's Industrial Works Department last year cracked down on the use of cyanide, following the arrest of an alleged serial killer accused of murdering her victims with the poison.
The case of "Am Cyanide" is still before the courts but police said they had found evidence she had poisoned 15 people over eight years in eight provinces, only one of whom survived.
Just like the bodies found in the Grand Hyatt on Tuesday, her alleged victims had black nails, hands and lips.
The stricter regulations mean that importers bringing the poison into Thailand for research or small-scale metal cleaning must be certified and report how much cyanide is their possession every three months.
Misuse of the toxin carries a penalty of three years in prison or a maximum fine of around $12,000.
Fears over tourism impact
Just hours after the bodies were found on Tuesday afternoon, Thailand's prime minister and interior minister rushed to the scene.
They were there to assess the potential damage to the country's precious tourism industry.
The government is ever conscious of bad publicity that could impact a sector that generates tens of billions of dollars a year.
Prime Minister Srettha Thavisin looked almost relieved as he declared the grisly crime was not terror-related, confirming that a meeting with Russia's energy minister due to be held at the same hotel was unaffected.
"This incident is not related to terrorism or a breach of security. Everything is fine," he said, despite the six bodies still lying in a hotel room nearby.
Bangkok's Deputy Police Chief Noppasin Punsawat later moved to reassure the public that the case appeared to be personal, and would not impact the safety of tourists.
"This case is a personal matter among these six individuals," he told reporters.
"It is not anything to do with a gang or organised crime here in Thailand."
Crimes like this are rare in Thailand but the tourism sector was shaken last October by a shooting spree close to the Grand Hyatt in which two foreigners died.
So, as police continue to investigate this bizarre and disturbing alleged crime, the government will likely continue to work on its damage control.
This story was originally published by ABC News.
Where to get help
Need to Talk? Free call or text 1737 any time to speak to a trained counsellor, for any reason.
Lifeline: 0800 543 354 or text HELP to 4357
Suicide Crisis Helpline: 0508 828 865 / 0508 TAUTOKO (24/7). This is a service for people who may be thinking about suicide, or those who are concerned about family or friends.
Depression Helpline: 0800 111 757 (24/7) or text 4202
Samaritans: 0800 726 666 (24/7)
Youthline: 0800 376 633 (24/7) or free text 234 or email talk@youthline.co.nz
What's Up: free counselling for 5 to 19 years old, online chat 11am-10.30pm 7days/week or free phone 0800 WHATSUP / 0800 9428 787 11am-11pm Asian Family Services: 0800 862 342 Monday to Friday 9am to 8pm or text 832 Monday to Friday 9am - 5pm. Languages spoken: Mandarin, Cantonese, Korean, Vietnamese, Thai, Japanese, Hindi, Gujarati, Marathi and English.
Rural Support Trust Helpline: 0800 787 254
Healthline: 0800 611 116
Rainbow Youth: (09) 376 4155
OUTLine: 0800 688 5463 (6pm-9pm)
If it is an emergency and you feel like you or someone else is at risk, call 111.