The Guardian - Police investigating the deaths of six people found in the room of a luxury hotel in central Bangkok say they believe a member of the group poisoned themselves and others using cyanide.

Initial tests had detected cyanide, a deadly chemical that interferes with the body’s ability to use oxygen, in a tea flask, six cups and in the blood of one of the dead men, police said. Based on interviews with a relative, they believe the deaths could be related to a business dispute.

The deceased were Vietnamese, and two had American citizenship, according to the Thai authorities, who said the FBI was assisting with the investigation.

Thai police said they believed the poisonings occurred on Monday afternoon, after the group ordered food and English tea to their room on the fifth floor of the Grand Hyatt Erawan hotel in Bangkok’s commercial district.

When room service staff arrived, they saw only one of the six, a 56-year-old woman, police said. She received the food and drinks and told hotel staff there was no need to serve the tea, according to Noppasin Poonsawat, the deputy commander of the Metropolitan Police Bureau.

Shortly afterwards, CCTV showed all six members of the group gathering outside the room and then entering, the last time they were seen alive.

“It all started after 13:57 [on Monday], after the hotel staff brought six teacups, a milk pot and two flasks into the room. We found cyanide on the six cups. According to CCTV, there was no one else – apart from the six members of the group – going inside the room, and none of them were seen coming out, after 14:17,” Noppasin said.

The group of three women and three men were found dead the following day, late on Tuesday afternoon. They were discovered by a member of housekeeping staff, after they were late to check out.

The plates of food they had ordered appeared to have gone untouched, and were still sealed in clingfilm. Their luggage was packed. Police said no illegal materials had been found inside their luggage, which was searched overnight.

The group were named by media as Vietnamese nationals Thi Nguyen Phuong, 46, her husband, Hong Pham Thanh, 49, Thi Nguyen Phuong Lan, 47, and Dinh Tran Phu, 37, and the US citizens Sherine Chong, 56, and Dang Hung Van.

Police are studying wider CCTV footage to piece together a timeline of their stay in Bangkok. The FBI was supporting the investigation as two of the individuals held American citizenship, the Thai prime minister, Srettha Thavisin, said.

Srettha said the case was not related to terrorism and that a meeting with the Russian energy minister due to be held at the same hotel on Wednesday was unlikely to be affected. “The incident is not related to terrorism or lack of security measurement, so I think everything will go as schedule,” he said.

Police had been searching for a seventh person who was included in the group’s booking but did not check in; however, they said on Wednesday the seventh individual was a younger sister of one of the women and she had returned home to Vietnam before the incident.

Police said two members of the group, the married couple, had loaned 10m baht ($278,025/£233,186) to another member of the group to invest in a business project to build a hospital in Japan, and there appeared to be a dispute over money. The couple ran a construction company in Vietnam.

Further information about the quantity of cyanide ingested is expected on Wednesday afternoon.

The US state department was “closely monitoring the situation and [we] stand ready to provide consular assistance”, a spokesperson said.

It is not the first high-profile killing to have involved cyanide in Thailand. Last year, a Thai woman accused of poisoning people with cyanide was charged with 14 counts of murder, in one of the country’s worst suspected serial-killing cases.