Doctors said tests on the bodies of all six people found dead in a luxury Bangkok hotel suite showed traces of cyanide in their blood.
Postmortem examinations revealed that there was “no other cause” that could explain their deaths other than cyanide.
But they are conducting further tests to determine the “strength” of the deadly chemicals and to check for other toxins.
Forensic investigators previously found traces of cyanide in teacups used by the victims, all of which were made in Vietnam. Police suspect that one of the dead was behind the poisonings and may have died from massive debt.
The victims' lips and fingernails had turned a deep purple, indicating a lack of oxygen, while their internal organs had turned “blood red” – another sign of cyanide poisoning, said Professor Konkiat Vongpaisansin of Chulalongkorn University's forensic medicine department.
Dr. Chanchai Sitiphunt, dean of the medical school, said they needed to find out how much cyanide was in the deceased's blood.
“But from what we found – from observation, from examining the internal organs, from finding cyanide in the blood during the screening test – there is no other cause of death that could have occurred other than cyanide,” he told reporters.
The body was found late Tuesday by a housekeeper at the Grand Hyatt Erawan hotel in the Thai capital.
Investigators estimate they had been dead between 12 and 24 hours by then.
The mystery surrounding this shocking discovery has captured the attention of the world.
Thailand's Prime Minister Sreta Thavisin has ordered an urgent investigation into the incident, stressing that the death was a “private matter” and that there was no suggestion of a public risk.
Since then, police have begun investigating what might have happened.
Authorities said two of the six victims had loaned “tens of millions of Thai baht” to the other deceased for investment purposes – worth about $280,000 (£215,000).
Earlier on Wednesday, Bangkok police deputy chief Gen. Noppasin Poonsawat told a news conference that the group checked into a separate hotel over the weekend and were given five rooms: four on the seventh floor and one on the fifth floor.
They were scheduled to check out on Monday but they didn't.
The four 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 US citizens Sherin Chong, 56, and Dang Hong Van, 55.
The U.S. State Department expressed its condolences and said it was “closely monitoring” the situation. Mr. Sreta said the FBI was assisting Thai authorities in their investigation.
