Police are looking for the source of the cyanide used in the killing of six Vietnamese people at a hotel in Bangkok early this week.
Police are investigating whether the cyanide was smuggled into the country or bought locally.
Police were also trying to contact the younger sister of one of the six victims. She left for Vietnam on July 10.
Police believe that Sherine Chong, one of the six people found dead at the Grand Hyatt Erawan Hotel in Bangkok on Tuesday evening, poisoned the others and then committed suicide, with a large debt issue being the possible motive.
A police source said investigators had invited Tien Thang Pham, the former husband of one of the six dead, for questioning at Lumpini police station on Wednesday.
The source said the man was on a trip to Japan and had a video call with his former wife before the killings occurred.
The source also said police had contacted the United States for more information about Sherine, who had American citizenship.
It was found that other Vietnamese in San Francisco had filed police complaints against her more than ten years ago.
They accused her of defrauding them by claiming she could help them secure American citizenship in exchange for money.
However, prosecutors in the United States decided to drop the case against her, the source said.
