Police are investigating the source of the cyanide used in the killing of six people of Vietnamese origin at a hotel in the Ratchaprasong area of Bangkok early this week.
Pol Maj Gen Witthawat Chinkham, commander of the Metropolitan Police Bureau's Division 5, said police are investigating whether the cyanide was smuggled into the country or bought locally.
He said police are awaiting the results of all forensic tests before deciding whether to call anyone in for further questioning.
Police were also trying to contact the younger sister of one of the six victims. She left for Vietnam on July 10.
Chanchai Sittipunt, director of Chulalongkorn Hospital, said on Thursday that autopsies on the bodies of the six dead had been completed.
He said that if the police do not seek any further information, the relatives will be allowed to collect them.
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 potential motive.
Pol Maj Gen Theeradet Thumsuthee, investigation chief at the Metropolitan Police Bureau (MPB), previously said at Lumpini police station that police had questioned the daughter of one of the victims, as well as other witnesses.
Their accounts were useful, he said.
"The case likely stems from a debt problem. There are no other possibilities. The culprit is among the six dead because they were the only people who entered the room. There were no others," Pol Maj Gen Theeradet said.
The six were found dead in a room on the fifth floor of the Grand Hyatt Erawan Hotel in downtown Bangkok on Tuesday evening.
Two had American citizenship and the others were Vietnamese nationals.
A seventh Vietnamese person had booked the room next door. Police believe she was a younger sister of one of the victims.
The woman left the country on July 10 and was unlikely to have had anything to do with the deaths.
Kornkiat Vongpaisarnsin, director of the autopsy centre at Chulalongkorn University's Faculty of Medicine, previously said an initial check of the blood samples of the six deceased had detected cyanide.
A police source said investigators had invited Tien Thang Pham, the former husband of Thi Nguyen Phuong Lan, 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 law enforcement officers in the United States for more information about Sherine, who had been granted 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 ultimately decided to drop the case against her, the source said.
