Traces of the rapid-acting, deadly chemical were found in the victims’ blood during autopsies and in the cups and thermoses found in the room at the luxury Grand Hyatt Erawan hotel where the dead were discovered late on Tuesday, according to police and a hospital.

The six had last been seen alive when food was delivered to the room on Monday afternoon.

The staff saw one woman receive the food, and security footage showed the rest arriving one by one shortly after.

There were no other visitors, no one was seen leaving and the door was locked.

A maid found them on Tuesday afternoon when they failed to check out of the room.

Interviews with relatives of the three women and three men who died revealed there had been a dispute over debt related to an investment, according to police, who said they were investigating how the cyanide was obtained.

The six were all of Vietnamese ethnicity, two of those US nationals.

Police said the US FBI had assisted with the investigation.

“We can assume that the six died from cyanide,” Chulalongkorn Hospital’s Kornkiat Vongpaisarnsin told reporters on Wednesday, adding that results of further tests would be available on Friday.

Their ages ranged from 37 to 56, according to Noppasin Punsawat, Bangkok deputy police chief.

A husband and wife among the dead had invested about 10 million baht ($A400,000) with two of the others, and that could be a motive, said Noppasin, citing information obtained from relatives.

The investment was meant to build a hospital in Japan and the group might have been meeting to settle the matter.

Police said one killed the rest but did not say which of the six was the suspect.

The US State Department said it was monitoring the situation and local authorities were responsible for the investigation.

Vietnam’s foreign ministry confirmed four of the dead were Vietnamese nationals and its embassy in Thailand was co-ordinating closely with authorities.

– AAP