The Office of Narcotics Control Board (ONCB) is working closely with the Food and Drugs Administration and Customs Department to find the source of diazepam -- an anti-anxiety medication that was detected in the fatal ketamine cocktail known on the streets as "k-nom pong".
At the same time, authorities are also looking into another ketamine concoction called "Talaysai", after a friend of a victim that had overdosed on k-nom pong told police they were warned that it is much stronger than k-nom pong.
Vichai Chaimongkol, secretary-general of the ONCB, said diazepam -- more commonly known by its brand-name, Valium -- is not available over the counter in Thailand, and lab tests showed that the diazepam found in the drug cocktail is "90% pure".
Imports of diazepam are supervised by the FDA, so the ONCB has asked the agency to forward a list of distributors for examination, he said.
The Customs Department has also been asked to examine imports of pharmaceuticals as part of the crackdown on the drug, which claimed the lives of at least seven people in Bangkok last week.
Experts determined the drug -- also known as "K-powdered milk" -- to be a combination of heroin, ketamine, methamphetamine and sleep medication, and authorities are trying to find where it is produced.
Mr Vichai said while the discovery of diazepam in the cocktail meant that it is unlikely the drugs were made by small-time dealers, as diazepam is too expensive to be used to "cut" harder drugs with.
"From our initial investigation, the diazepam is likely to have been smuggled from abroad, or smuggled out of pharmacies," he said.
Pol Lt Gen Phukphong Phongpetra, commissioner of Metropolitan Police Bureau, on Thursday vowed to make progress in the investigation in a week.
He said police have found evidence linking the deaths and arrests last week, which occurred under the jurisdictions of Wat Phraya Krai, Sutthisan and Sai Mai police stations.