Last spring, Chinese customs agents seized 70 kilograms of the narcotics fentanyl and acetyl fentanyl hidden in a cargo container bound for Mexico.

The synthetic opium-like drugs were so potent that six of the agents became ill after handling them. One fell into a coma.

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The cargo had traveled through five freight forwarders before reaching customs, obscuring its exact origins, according to an internal U.S. Drug Enforcement Administration intelligence briefing reviewed by The Wall Street Journal.

One thing is clear: The shipment and a host of others, detailed in the DEA briefing, court documents and interviews with government officials in multiple countries, are part of a vast drug-distribution network beginning in China that feeds lethal fentanyl to the Americas.

The network often avoids efforts to stop it by trading not only in finished fentanyl but related products subject to little or no regulation in China or internationally. These include some copies of fentanyl known as analogs, as well as the chemical ingredients and pill presses used to produce the drug, according to the documents and interviews.

Click for more from the Wall Street Journal.