Based on the initial post-mortem examination, they say there is "no other cause" that would explain their deaths "except for cyanide", reported BBC.

Doctors are trying to conduct further tests 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 them of Vietnamese origin, reported BBC.

Police suspect that one of the dead persons was behind the poisoning episode.

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, Professor Kornkiat Vongpaisarnsin of the Department of Forensic Medicine at Chulalongkorn University told BBC.

The housekeepers found the bodies in a hotel room of the luxury hotel in Bangkok.

Investigators believe that they had been dead for 12 to 24 hours period.

Two of the six victims had loaned "tens of millions of Thai baht" to another of the deceased for investment purposes, authorities were quoted as saying by BBC.

Ten million baht is worth close to $280,000 (£215,000).

The four victims who have been identified as Vietnamese nationals are Thi Nguyen Phuong, 46, her husband Hong Pham Thanh, 49, Thi Nguyen Phuong Lan, 47, and Dinh Tran Phu, 37.

The other two dead people were identified as American nationals Sherine Chong, 56, and Dang Hung Van, 55.

The US State Department said it is aware of the death of two American nationals in Bangkok.

"We are aware of the reports of the deaths of two U.S. citizens in Bangkok, Thailand. We offer our sincere condolences to the families on their loss. We are closely monitoring the situation and stand ready to provide consular assistance to those families," US State Department spokesperson Matthew Miller told a briefing Tuesday.

"Whenever a U.S. citizen dies in a foreign country, local authorities are responsible for determining the cause of death," he said.

"We do reach out to local authorities often to communicate them about – to communicate with them when it involves the death of a U.S. citizen, and we will certainly be doing so here. But I would defer to the local authorities for any further comment on what I believe is an ongoing investigation," he said.