A "technical error" in field testing had led to the false claim that 11.5 tonnes of ketamine had been seized at a warehouse in Chachoengsao province, Justice Minister Somsak Thepsuthin explained on Tuesday.
Mr Somsak made the admission at a media briefing called to clear up the matter, at the Justice Ministry, in the presence of Office of the Narcotics Control Board secretary-general Wichai Chaimongkol.
The 475 sacks containing a white, granular substance, 11.5 tonnes in all, were seized by ONCB officials during a raid on a warehouse at tambon Tha Kham in Bang Pakong district, Chachoengsao province, on Nov 12.
Samples taken from sacks at the warehouse turned the ONCB's testing fluid purple. It was declared to be ketamine, and the seizure triumphantly claimed to be Thailand's largest ever.
But after the sacks were taken to the ONCB Region 1 office in Lat Lum Kaew district, Pathum Thani province, laboratory tests on samples from 66 sacks found it was not ketamine at all, but trisodium phosphate - a compound used legally as a food additive and stain remover.
Mr Somsak said a "technical error in the field" led to the assumption it was ketamine. The ONCB was unaware that trisodium phosphate would also turn the testing fluid purple, not only a drug like ketamine.
"No matter, we have admitted the mistake, and it may not be corrected in the short term," he added.
Asked if the blunder could affect the credibility of the Justice Ministry and the ONCB, Mr Somsak said he accepted all criticism.
Mr Somsak said samples from 66 sacks had been tested and turned out to be trisodium phosphate. Testing of the remaining sacks was continuing.
The minister said that to make the matter clear, he would ask the police Forensic Science Division, the Department of Medical Science and the ONCB to make lab tests. The results should be known this week. Other agencies could also take part if they wanted to.
Mr Somsak said the seized sacks of the chemical had not disappeared or been moved to anywhere else. On a suggestion that the justice minister was required to sign to certify the seized items, Mr Somsak said there was no such a regulation.
"I accept the fact it might have been premature to hold a press conference to announce the seizure of a substance suspected to be a kind of drug.
"But in this case, the ONCB had been informed of the seizure of ketamine in Taiwan, investigated and found an undeniable link to it. It would have been a mistake if I did not make it public," he said.
"The United Nations Office on Drugs and Crime (UNDC) said this also happened two or three times in other countries. This was the first time in Thailand. Moreover, on the day I held the press conference, I did not say it was 100% ketamine," Mr Somsak said.
Mr Somsak said the error caused misunderstanding and led some people to spread misinformation on social media, causing damage to other people who were not involved. He did not elaborate.
Therefore, he had set up a committee, chaired by the justice permanent secretary, to work with the Department of Special Investigation and representatives of the Council of State, the Office of the Attorney General and the Technology Crime Suppression Division to investigate and take legal action against those who published the false information.
Mr Wichai said the testing of the substance taken from the sacks at the warehouse was done by the police Forensic Science Division and the ONCB. The FSD's result was orange, while the ONCB's was purple. Both colours indicated the substance was an illicit drug. Purple was not only for ketamine, but also a few other drugs, he added.
He said trisodium phosphate was not a narcotic, it was use in the food industry and in production of milk, to remove discolouring.
Authorities would find out where the substance found in the Chachoengsao warehouse came from, and for what purpose. It was believed it could be used to conceal illicit drugs, including ketamine, Mr Wichai said.
The ONCB chief said the man who rented the warehouse to store the sacks had not been arrested because he fled before the raid. Authorities were compiling a case to apply for a court warrant for his arrest in connection with the seizure of ketamine in Taiwan.
He said the ONCB would next week organise a seminar on narcotic drugs which test purple. It would be attended by officials from the US Drug Enforcement Administration, UNODC and other agencies concerned with illicit drugs.
On Monday, Atchariya Ruangrattanapong, chairman of the Crime Victims Assistance Club, filed a complaint with Anti-Corruption Division police against the justice minister and the ONCB secretary-general, accusing them of giving false information to the public over the seizure.