The case had much progressed and pending probe for only 5-6 additional points, he said, so far police interviewed 15 witnesses and seven doctors leading to the arrest warrants for sisters Prapa Palsingh, Rattana Singsajjakul and Naowarat Koker.

Three sisters charged with conspiring to murder – after their brother accused them of drugging their mother over a long period, plus fabricating her will and embezzling Bt2 billion in assets – have been released on bail.

They denied conspiring to commit premeditated murder, but said they would only discuss the case in court.

Pol Lt-General Sanit Mahathaworn of the Metropolitan Police Bureau said a panel had been set up to look into the case, which eventually issued arrest warrants for them in December last year after it found the allegations to had merit. Sanit said police were confident they had a solid case because there was a motive to commit the crime, and evidence showed the mother had died as a result of consuming the drug cocktail over a long period of time.

Police were gathering evidence to press charges against others allegedly involved in the crime, he said, adding that all culprits would be brought to justice in a transparent manner.

“Money cannot buy me,” he said.

Kamol Pichitsingh, an Indian businessman, said his mother Rajit Pichitsingh died of heart failure in July 2013. He said his younger sister Prapa Palsingh had been taking care of their mother since she suffered a stroke in 2005. The businessman also claimed that two other brothers had sided with Prapa and his two other sisters Rattana Singsajjakul and Naowarat Koker because they had refused to help Rajit.

Kamol said before Rajit died, he had visited her a few times and noticed she appeared dazed and heavily drugged. He asked the housekeeper what drugs she was being given and was told his sister kept their mother’s medication in a case.

He took the case to the private hospital treating his mother to get the drugs identified and found that some of them had not been prescribed, including muscle relaxants and sleeping pills. At the hospital, he was told that taking these drugs in combination over a long period of time could kill a person.

He said he petitioned the Crime Suppression Division to remove his mother from her Bangkok residence but the division ignored his complaint.

He then filed a complaint at Lumpini police station and asked police for the drug report he had submitted earlier and evidence to be returned.

Kamol said the day his mother died he asked the hospital to carry out an autopsy over the objections of his sisters. An emergency-room doctor took a blood sample from her spine and sent it to Ramathibodi Hospital to be tested. The results showed that her blood contained massive amounts of sleeping pills and painkillers.

He said his mother’s will said that all of her assets would be left to his female siblings.

Kamol said there were several irregularities in the will. He claimed officials had committed wrongdoing, adding there was no reason that she would have altered the will outside her home. Rajit could not speak Thai and there was no record of translators being present when the will was made. On the day the will was read, the three sisters did not appear with the rest of the family.

Pol Major Thanawat Leepong, the Crime Suppression investigator who accepted a complaint from Kamol in 2013, said he did not take action because the businessman had already filed complaints with local police and filed petitions with the courts.

He said Kamol asked him to press charges against his siblings for attempted murder, fabricating documents, giving false statements and embezzlement.

Thanawat said he sent the case to Lumpini police, which had jurisdiction, 18 days after the complaint was filed. “I would not risk my job by doing such a thing. I have never met nor I do know either party in the case. Though there are allegations I took a kickback, I have never done that. This case is simply beyond my authority because the local police had accepted the complaint,” he said.