Cyanide poisoning was likely the cause of the deaths of six foreigners whose bodies were found in a room in a plush Bangkok hotel, with the suspected killer among the dead, Thai police said on Wednesday

The rapid-acting, deadly chemical was found on drinking glasses and a teapot in the room at the luxury Grand Hyatt Erawan hotel, and interviews with relatives of the dead revealed there had been a dispute over debt related to an investment, police said.

The six were all of Vietnamese ethnicity, two of those U.S. nationals, and were found dead late on Tuesday. 

The U.S. Federal Bureau of Investigation had assisted police with the investigation, police said.

"We found cyanide in the teacups, all six cups we found cyanide," Trirong Phiwpan, Commander of the Thai police evidence office, told a press conference.

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

The results of an autopsy were expected within the next day, police said.