Cyanide has been found in the blood of all six people who died in a luxury hotel suite in Bangkok, say doctors after examining their bodies.
Based on the initial post-mortem examination, they say there is "no other cause" that would explain their deaths "except for cyanide".
But further tests are being carried out to determine the "intensity" of the deadly chemical and to rule out any other toxins.
Forensic investigators had earlier found traces of cyanide on the teacups used by the victims, all of whom are of Vietnamese origin including two with dual US citizenship. Police suspect that one of the dead was behind the poisoning and was driven by crushing debt - but have not said who.
The victims' lips and nails had turned dark purple indicating a lack of oxygen, while their internal organs turned "blood red", which is another sign of cyanide poisoning, said Professor Kornkiat Vongpaisarnsin of the Department of Forensic Medicine at Chulalongkorn University.
Doctor Chanchai Sittipunt, the dean of the Faculty of Medicine, said they still needed to find out how much cyanide was in the blood of the deceased.
"But from what we have detected - from observation, from internal organ check, from finding cyanide in the blood during the screening test - there is no other cause that would be the factor that would cause their deaths, except for cyanide,” he told reporters.
The deceased were found by housekeepers at the Grand Hyatt Erawan hotel in the Thai capital late on Tuesday.
Investigators believe they had been dead for between 12 and 24 hours by then.
The mystery around the shocking discovery made international headlines.
Thailand's Prime Minister Srettha Thavisin ordered an urgent investigation into the case, stressing that the deaths were the result of a "private matter", and there was no suggestion of public danger.
Police have since begun to piece together what might have happened.
Two of the six victims had loaned "tens of millions of Thai baht" to another of the deceased for investment purposes, authorities said. Ten million baht is worth nearly $280,000 (£215,000).
Earlier on Wednesday, Deputy Bangkok police chief Gen Noppassin Poonsawat told a press conference the group checked into the hotel separately over the weekend and were assigned five rooms - four on the seventh floor, and one on the fifth.
They had been scheduled to check out on Monday but failed to do so.
Four of the victims are Vietnamese nationals Thi Nguyen Phuong, 46, her husband Hong Pham Thanh, 49, Thi Nguyen Phuong Lan, 47, and Dinh Tran Phu, 37.
The other two are American citizens Sherine Chong, 56, and Dang Hung Van, 55.
The US state department has offered its condolences and said it is "closely monitoring" the situation. The US Federal Bureau of Investigation is assisting Thai authorities in the investigation, Mr Srettha said.
What do police suspect happened?
Police say on Monday afternoon all six victims gathered in the room on the fifth floor.
The group ordered food and tea, which was delivered to the room around 14:00 local time (08:00 BST) and received by Ms Chong - who was the only person in the room at the time.
According to the deputy police chief, a waiter offered to make tea for the guests but Ms Chong refused this. The waiter recalled that she “spoke very little and was visibly under stress”, authorities said.
The waiter later left the room.
The rest of the group then began streaming into the room at various points, between 14:03 and 14:17. No one else is believed to have entered the room apart from the six inside and police have said the door to the room was locked from within.
Police say there were no signs of a struggle, robbery or forced entry. They later found traces of cyanide in all six tea cups.
Pictures released by the police show plates of untouched food left on a table in the room, some of them still covered in cling wrap.
There was a seventh name on the group's hotel booking, whom police identified as the younger sister of one of the victims. She had left Thailand last week for the Vietnamese coastal city of Da Nang and is not involved in the incident, police said.
Relatives interviewed by the police said Thi Nguyen Phuong and Hong Pham Thanh, a couple, owned a road construction business and had given money to Ms Chong to invest in a hospital building project in Japan.
Police suspect that Mr Tran, a make-up artist based in Da Nang, had also been “duped" into making an investment.
Mr Tran's mother Tuý told OceanNewsUK that he had travelled to Thailand on Friday and had called home on Sunday to say he had to extend his stay until Monday. That was the last his family had heard from him. She rang him again on Monday but he did not answer the call.
Ms Chong had hired Mr Tran as her personal make-up artist for the trip, one of his students told OceanNewsUK. Mr Tran's father, Phu, told Vietnamese media that his son was hired last week by a Vietnamese woman to travel to Thailand.
The six bodies were discovered one day after Thailand expanded its visa-free entry scheme to travellers from 93 countries and territories to revitalise its tourism industry.
The Grand Hyatt Erawan is located in a prime location popular with tourists - however the area has also seen high-profile crimes in recent years.
Last October, a 14-year-old boy shot and killed three people at the Siam Paragon mall, which is just a few hundred metres down the road from the hotel.
The hotel also sits opposite the Erawan Shrine, which was hit by a bomb blast in 2015 that killed 20 people.
Prime Minister Srettha himself visited the hotel on Tuesday and reassured the public that Thailand has put in place security measures to keep tourists safe.
Tourism is one of its biggest industries and has just begun to recover from the impact of the coronavirus pandemic.
Thailand has always been a top destination for the budget tourist - from students to thrifty backpackers - but it is increasingly trying to attract well-heeled travellers.
What is cyanide and how dangerous is it?
Cyanide is a rapidly-acting, highly toxic chemical that is potentially deadly. Low levels of cyanide occur in nature and in products we eat and use. But in larger doses it is a notorious poison, and has been used as a chemical warfare agent because of its fast-acting and highly lethal properties.
It can occur as a colourless gas or liquid or in crystal form. People can be exposed to cyanide by breathing it in, absorbing it through the skin, or eating food or liquids that contain it.
When consumed in large amounts, cyanide can lead to lung injury, coma and death within seconds, according to the Centers for Disease Control and Prevention (CDC).
Even in smaller doses, cyanide is still very harmful, causing chest pain, nausea, shortness of breath and vomiting.
Cyanide can produce a “bitter almond” smell but not everyone can detect this and it doesn’t always give off an odour.
