Thai Prime Minister Srettha Thavisin told the press on Wednesday that the FBI would be joining local authorities to investigate the deaths of the six guests at Grand Hyatt Erawan hotel in downtown Bangkok.
He said the FBI's involvement was due to the presence of two Vietnamese-American individuals among the victims, The Strait Times reported.
Earlier, Thai authorities confirmed that the victims were two Vietnamese-Americans and four Vietnamese nationals, aged between 37 and 56.
State Department spokesman Matthew Miller said earlier that U.S. officials are aware of the reports of the deaths of two U.S. citizens in Bangkok.
"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," he said, as cited by AP.
The bodies of the victims, three men and three women, were found in a locked room on the fifth floor of the hotel. Their deaths may have been caused by poisoning, police and officials said.
Hotel staff alerted the police around 5:30 p.m. on Tuesday, after using a back door to enter the room for inspection, as the group had been overdue for check-out by about 24 hours.
"We are still waiting for forensic evidence and the initial hypothesis suggests this is a personal case," Bloomberg quoted Srettha as saying.
Thai authorities have not yet concluded whether this was a case of suicide or homicide. Initially, the police ruled out the possibility of a murder-robbery and do not believe it was related to national security or terrorism.
Bangkok police commissioner Thithi Saengsawang said at the press briefing that the victims were not dead from suicide but from the killing of others.
According to the hotel's records, seven people booked five rooms on the fifth and seventh floors at the hotel, but only five checked in for the five rooms. Four rooms on the seventh floors were scheduled for check-out on July 15, and their bags were packed. Their bodies were discovered by hotel staff in a fifth-floor room on the day.
The victims ordered food at around 1 p.m. on Monday, but the food appeared to be uneaten, but six cups were found empty and drinks appeared to be consumed, Vietnam News Agency reported.
According to Thiti, none of the bodies showed any signs of suffering physical violence. Only one victim had a facial wound, and police assumed the person fell onto a hard object.
Authorities are currently searching for the seventh person who was part of the hotel booking, and is now a suspect.
The investigation agency is examining the closed-circuit television footage since they touched down in Thailand to clarify the case.
The Vietnamese Embassy in Thailand is working with competent authorities in Thailand to clarify the case and carry out necessary citizen protection measures.