Thai police have found traces of cyanide in the drinking cups of six people found dead in a luxury hotel in central Bangkok.

The bodies of three men and three women were found by a maid in one room of Bangkok’s Grand Hyatt Erawan hotel on Tuesday.

Police official Thiti Saengsawang said earlier that all six of those who died – four Vietnamese nationals and two Americans of Vietnamese descent – had checked in at two separate times after arriving on Saturday and Sunday.

The group checked into five rooms. Their bodies were found in one room, and there were no signs of struggle, Thiti said.

“This was not self harm but someone caused the deaths,” he said.

“We are tracing every step since they got off the plane.”

Police officers found the bodies on Tuesday after a call from hotel staff to report mysterious deaths about 5.30pm, Thai police said.

The group was supposed to have checked out earlier, and their luggage had already been packed, Thiti said. The bodies were found by a worker who went to the room after they failed to check out, and found it locked from the inside.

Food ordered from room service on Monday afternoon – the last time the group was seen alive – was found uneaten in the room.

Thiti initially said that drinks delivered with the food had been consumed, and an “unidentified substances” left in the bottom of all six cups.

On Wednesday, Bangkok police forensic division chief Trairong Piwpan said traces of cyanide had been found in the cups and a teapot in the room.

Police were initially looking for a seventh person. On Wednesday, they said they believed the suspected killer was among the dead.

“We found cyanide in the teacups, all six cups we found cyanide,” Trirong said.

“After staff brought tea cups and two hot water bottles, milk and tea pots … one of the six introduced cyanide.”

Initial autopsy results are not expected until Thursday.

Bangkok deputy police chief Noppasin Punsawat said police – citing information from relatives of the dead – suspected the motive for the deaths might have been a dispute between a husband and wife about investments.

Thai prime minister Srettha Thavisin, who visited the hotel late on Tuesday with senior police officials, ordered a swift investigation into the deaths.

“The prime minister has ordered all agencies to urgently take action to avoid impact on tourism,” the government said.

A US State Department spokesperson said it was “closely monitoring the situation and [stood] ready to provide consular assistance”.

The Grand Hyatt Erawan, which has more than 350 rooms and is in a popular tourist district in the Thai capital known for luxury shopping and restaurants, did not respond to calls or an email seeking comment.

-with AAP