The cargo had been procured by a broker based in territory controlled by the United Wa State Army, a militia that for years has been accused of funding itself through drug sales.

But local authorities had not heard of propionyl chloride. It is not one of the 30 precursor chemicals scheduled by the International Narcotics Control Board (INCB) for use in manufacturing illicit narcotics or psychotropic substances.

Nor had there been any apparent attempt to conceal the cargo though the corrugated shipping container had taken an unusual route thousands of miles around Asia, rather than overland through China.

The propionyl chloride departed China's coastal province of Jiangsu, north of Shanghai, on a ship bound for the Thai port city of Laem Chabang near Bangkok. From there, the chemicals were transported north by land until they reached the Lao district of Huay Xai, just across the Mekong River from Thailand.

Laotian authorities decided to call Jeremy Douglas for advice. Douglas is the regional representative for the United Nations Office on Drugs and Crime (UNODC), and it's his job to help governments throughout East Asia and the Pacific combat transnational criminal activity. In the lower Mekong, that often means drug trafficking.

Douglas was astonished.

He urged the Laotians to seize the chemicals because he knew propionyl chloride can be used to make fentanyl, a powerful and dangerous synthetic opioid that's ravaged the United States in recent years, and ephedrine, a key ingredient in methamphetamine.

Propionyl chloride is not on the INCB list because it has plenty of legitimate uses, such as the production of agricultural chemicals and pharmaceuticals. However, the INCB recommends nations subject it to "Special surveillance."

News of the seizure was kept under wraps until April this year, when Douglas and Thai authorities presented it at a virtual drug conference organized by the United Nations Global Drug Commission.

Laos authorities had stumbled upon a smoking gun, a major piece of evidence that likely explained how the kingpins behind Asia's multibillion-dollar synthetic drug industry had been outfoxing the Mekong's security forces.

They were employing ingenuous chemical engineering, using a variety of unregulated chemicals, to make synthetic narcotics.

"These are very creative people," Douglas told the UN conference.

"Fundamentally, they are innovators. They are problem solvers."

The working theory

Authorities seized a record 175 tons of meth in 2020 throughout East Asia and Southeast Asia, a new record despite the Covid-19 pandemic, according to preliminary UNODC data.

Drug prices continued to drop, meaning these major busts are not materially affecting the overall supply of drugs in the region.

But seizures of ephedrine, pseudoephedrine and phenyl-2-propanone (P2P), the most common chemicals used to make meth, basically dried up.

Douglas said authorities only seized 600 kilograms of ephedrine and 10 million tablets of pseudoephedrine, a "tiny amount" compared to the level of meth nabbed by authorities.

That left experts with a puzzling question: How was the meth being made?

If illicit drugs were being seized in record numbers, authorities should have found a higher volume of the chemicals to make them, too.

"Like buying the flour to make a pie crust"

Experts floated the theory that cartels were importing chemicals like propionyl chloride and employing world-class chemists to produce their own ingredients to make meth , like buying the flour to make a pie crust instead of just purchasing a pre-made one.

The law enforcement community often calls these chemicals "Pre-precursors" or "Non-scheduled-precursors."

They are made and sold legally but diverted for illicit use at some point in the supply chain.

Some pre-precursors like propionyl chloride have legitimate chemical uses besides illicit drug manufacturing. Other so-called "designer precursors" are synthesized so they're chemically distinct enough to avoid government oversight, but serve no known purpose other than making narcotics.

Trying to regulate these chemicals often resembles a game of whack-a-mole. By the time a government has gone through the bureaucratic or legal process to regulate one, another new one has appeared.

However, despite the seemingly never ending flow of newly developed pre-precursors, converting pre-precursors into the ingredients for synthetic drugs is a technically complex process that involves expert chemistry.

"Staggering volume"

Douglas said his office knew various pre-cursors were being confiscated across the Mekong, but the staggering volume of propionyl chloride seized in Laos all but confirmed their suspicions that illicit narcotics manufacturers were using this process.

"In a sense the seizure confirmed what we and others had suspected and for the past few years: that pre-precursors are a playing a major role in the regional drug trade," Douglas said.

"Organized crime are effectively working around controls on traditional precursors."

Video | Propionyl chloride can be used to make fentanyl, a powerful and dangerous synthetic opioid that's ravaged the United States in recent years.