A diplomatic row has frozen U.S.-Mexican efforts to target drug cartels. American officials say illicit fentanyl from labs in Mexico is driving a surge in overdose deaths.

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A top official with the U.S. Drug Enforcement Administration tells NPR efforts to target drug cartels in Mexico have unraveled. The culprit is a diplomatic route that's frozen joint investigations and intelligence sharing between the two countries. This comes at a time when cartels are shipping more and more fentanyl into the U.S., driving a record spike in overdose deaths. NPR addiction correspondent Brian Mann reports.

BRIAN MANN, BYLINE: Last year overdose deaths in the U.S. surged in a way no one's ever seen, killing more than 90,000 Americans. Matthew Donahue is head of operations for the DEA, which means he directs U.S. efforts to curb drug trafficking around the world. He says illegal labs run by cartels in Mexico are the major source of fentanyl and methamphetamines driving the epidemic.

MATTHEW DONAHUE: It's a crisis. It's a national security crisis. It's a national health threat, national safety threat.

MANN: But Donahue says efforts to fight drug cartels and target their operations inside Mexico have broken down because of a collapse in trust and cooperation between law enforcement and militaries in the two countries.

DONAHUE: We're willing to share with our counterparts in Mexico, but they themselves are too afraid to even engage with us because of repercussions from their own government if they get caught working with the DEA.

MANN: The crisis began last October, when federal agents in California did something unprecedented.

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UNIDENTIFIED REPORTER: An unusual arrest at LAX last night. The DEA arrested one of Mexico's top generals, Salvador Cienfuegos. Cienfuegos was arrested on drug trafficking and money laundering charges.

MANN: That report from CBS News. Cienfuegos was also Mexico's former defense secretary. Without first notifying their Mexican counterparts, the DEA detained Cienfuegos, accusing him of working for one of Mexico's deadliest cartels. Under pressure from Mexico, then-Attorney General William Barr backpedaled, dropping all charges and releasing Cienfuegos. But experts say the diplomatic damage was done.

FALKO ERNST: There's always been a very high level of mistrust between both sides, right?

MANN: Ernst Falko (ph) is an analyst with the International Crisis Group based in Mexico City. Before Cienfuegos' arrest, he says, U.S. law enforcement was able to target drug cartels inside Mexico with the help of a handful of trusted elements within the Mexican military and police. Now Falko says even those fragile links are broken.

ERNST: Operations there have pretty much been paralyzed, basically. So what the U.S. has built up in terms of good relationships with the parts of the Mexican state have pretty much gone.

MANN: In response to the Cienfuegos arrest, Mexican lawmakers approved a measure sharply restricting U.S. drug operations inside Mexico. The law also requires Mexican officials to share any intel the U.S. provides about the cartels with other agencies, including agencies the U.S. doesn't trust. As a result, information sharing and joint investigations ground to a halt. The Mexican government declined NPR's requests for interviews for this story, nor did they reply to questions submitted to multiple agencies within the Mexican government. The DEA's Matthew Donahue says the winners in all this are the drug cartels.

DONAHUE: They do not fear any kind of law enforcement inside - or military inside Mexico right now.

MANN: Sources in the U.S. tell NPR this breakdown makes it harder to track fentanyl shipments and other drugs as they cross the border, bound for cities and small towns across the U.S. This comes at a moment when the Biden administration is dealing with an escalation in the number of migrants on the U.S.-Mexico border while also scrambling to stem an overdose crisis that's killing 240 Americans a day. A White House official told NPR drug interdiction will be the subject of talks between the two countries soon. But Cecilia Farfan-Mendez, an expert on U.S.-Mexico security cooperation at UC San Diego, says restoring trust and cooperation won't be easy, especially with Mexican officials focused on elections next month.

CECILIA FARFAN-MENDEZ: With the elections coming up, it's - my expectation is that there's not going to be a lot of attention to what the U.S. would like to do and how to enhance that cooperation.

MANN: Mexican President Andres Manuel Lopez Obrador has also voiced skepticism of the old drug war model that targeted cartel kingpins. While that strategy led to arrests of high-level traffickers and produced splashy headlines, critics in the U.S. and Mexico say it never significantly slowed the flow of drugs into the U.S.

Brian Mann, NPR News.

(SOUNDBITE OF BJORK SONG, "PAGAN POETRY") Transcript provided by NPR, Copyright NPR.

Tags: drug cartels  Mexico