Mexico's Pacific port city of Manzanillo is on the front line of the fight against fentanyl -- a drug blamed for tens of thousands of deaths in the United States.

Since last year, agents at the port have been tasked with finding and seizing precursor chemicals used in the production of the synthetic opioid 50 times more potent than heroin and much easier and cheaper to produce.

This on top of the methamphetamines, cocaine and marijuana they were already on the lookout for among the 3.5 million ship containers that pass through Manzanillo every year.

"It's not that (seizures of precursor chemicals) didn't happen before. Before 2022 they weren’t regulated," Captain Luis Martinez of the Mexican navy in charge of drug searches told AFP on a recent tour of the port.

Under a new law passed last year by Congress, a search alert is in place at the port for certain chemicals that can be used in the production of fentanyl -- the leading cause of deaths among Americans aged 18 to 49 years.

Deaths from fentanyl overdoses in the United States rose from 69,000 in 2020 to 110,000 in 2022, according to the US Centers for Disease Control and Prevention.

As the drugs' addictiveness became increasingly clear, the United States pressured China -- the chief source of fentanyl -- to ban exports, which it did in 2019.

But China is still a major producer of precursor chemicals, which are shipped to Mexico and Central America, where cartels produce fentanyl for smuggling into the United States.

- Mexican cartels -

"The two drug cartels that are responsible for the influx of fentanyl into the United States, the Sinaloa and Jalisco cartels, work with chemical companies based in the People's Republic of China to get their raw materials," US Drug Enforcement Agency Administrator Anne Milgram said recently.

In Mexico, navy agents at Manzanillo told AFP they work on their own, without technical assistance from the United States.

It is dangerous work.

In May this year, Manzanillo's deputy customs director Sergio Emmanuel Martinez was assassinated just a fortnight after taking office. 

Mexican President Andres Manuel Lopez Obrador said the murder was a reaction to his government's anti-drug efforts.   

"We are fighting drug trafficking, preventing the introduction of these chemicals, destroying laboratories, doing our job," said the president. 

- Meth in tequila -

Agents can only do so much with their limited resources pitched against the ingenuity of traffickers.

Earlier this year, the navy said it had intercepted methamphetamines mixed into more than 8,000 liters of bottled tequila.

"The substance was diluted," said Martinez. 

Cocaine mixed with "organic fertilizers" was also recently seized, he added.

"In 2021 we seized 1,712 kilos of marijuana destined for Chile" that were hidden in containers filled with building bricks. 

According to the latest available data, almost 385 tons of chemicals used in the production of synthetic drugs, including fentanyl, were seized from December 2018 to September of last year, as well as 356 tons of ready-made fentanyl.

Once seized, the chemicals are broken down to their base molecules, to "guarantee it can no longer be used for the manufacture of synthetic drugs," said Martinez. 

The effort remains an uphill battle.

"We cannot inspect them all," said the captain of the mountains of containers passing through the port.

"We prioritize according to the level of risk," he said.

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