Mark Smith, special to ProPublica

In early 2011, the Drug Enforcement Administration obtained a rare and highly valuable piece of intelligence about the leaders of the Mexico-based Zetas cartel, one of the most powerful, and impenetrable, drug organizations in the world.

An agent in Dallas had persuaded the cartel’s leading cocaine distributor in East Texas to hand over trackable cellphone identification numbers for the group’s most wanted kingpins, in particular Miguel and Omar Treviño, a murderous pair of brothers whose viciousness had earned them top spots among the DEA’s most-wanted.

It was an intelligence coup, the kind of information that comes along once in a very lucky career. With those numbers, authorities could track the brothers’ movements and ultimately capture them. But the DEA made a decision with fatal consequences. Against the wishes of the lead agent on the case — whose informant specifically warned of the potential for bloodshed — the DEA told a Mexican federal police unit with a long history of leaking to traffickers that it had the information.

How the U.S. Triggered a Massacre in Mexico The inside story of a cartel’s deadly assault on a Mexican town near the Texas border — and the U.S. drug operation that sparked it. Read the story. Email Updates Sign up to get ProPublica’s major investigations delivered to your inbox.

Within days, the Zetas were, in turn, told that the DEA was onto their leaders. The Treviño brothers guessed immediately which of the cells in their organization had betrayed them and began hunting for the snitches. When the suspected traitors couldn’t be found, the traffickers went after anyone connected to them.

Dozens, possibly hundreds, of people were killed and kidnapped in and around Allende, a quiet ranching town in the northern Mexican state of Coahuila, about 40 minutes from the U.S. border. Zetas gunmen grabbed a 15-year-old high school football player, who was hanging out with friends whose parents ran a health club where one of the suspected snitches lifted weights. They took an 81-year-old woman, as well as her 6-month-old great-grandson. One family lost nearly 20 members.

Black clouds spewed from a local ranch where the cartel turned one building into a makeshift crematorium to burn the bodies of those they had killed.

For years, Mexican authorities did next to nothing to investigate the massacre. Meanwhile people in Allende, understandably distrustful of the authorities sworn to protect them, kept their mouths shut.

Tragically, that outcome has become all too familiar in Mexico, where impunity is a national scourge. Homegrown corruption, greed and fear have bred an epidemic of virtually unchallenged violence. What makes this case different is that the DEA lit the fuse that triggered the slaughter, then stood mutely by — as if it had played no role. DEA officials knew almost immediately that innocent lives had been lost as a result of sharing the intelligence with Mexico. The agency’s response then — and in the years since — nothing.

It didn’t demand answers from its Mexican counterparts, or suspend cooperation with the Mexican police until it could determine how the information was leaked. It didn’t conduct an internal investigation into the decision to share the intelligence or reassess its own rules for giving sensitive information to Mexico. It didn’t report the violence to superiors at the Justice Department or to overseers on Capitol Hill.

And, perhaps underscoring the perception that the lives destroyed were in some way acceptable collateral damage in the war on drugs, it didn’t offer to provide any assistance to those victimized by the leak or resources to help identify and arrest the perpetrators.

I’ve spent most of the last year investigating and documenting the attack on Allende, recording detailed, often gut-wrenching, accounts from those who lived through it and those whose actions helped cause it for ProPublica and National Geographic. Dozens of people in Allende agreed to speak to me on the record, many of them talking publicly for the first time and at great personal risk. Even the former Zetas-turned-informants spoke at length about their roles and their devastating consequences. The assistant U.S. attorney on the case described himself as “devastated.” And eventually, the DEA agent who led the investigation discussed, at times emotionally, his part in the tragedy.

But when presented with this array of voices and evidence, DEA officials refused to explain what, if anything, the agency had done to respond to the massacre. Spokesman Russ Baer would only say that the agency placed blame squarely on the Treviño brothers: “They were killing people before that happened, and they killed people after the numbers were passed.” He told me I needed to be clear on one thing: “This is not a story where the DEA has blood on its hands.”

That’s technically true, and sadly seems by design. Because of the way Mexico’s drug war is fought, the United States plays a leading role — providing training, equipment and intelligence to security forces with reputations for collaborating with traffickers — without sharing responsibility for the fallout.

Mark Smith, special to ProPublica

Some Mexican counternarcotics units or programs — including the one implicated in the Allende massacre — wouldn’t exist if it weren’t for the United States. American taxpayers have pumped hundreds of millions of dollars into Mexico’s counternarcotics programs over the years. But other than vague lists of kingpins who have been arrested and the occasional made-for-TV photo op of seized drugs, there is almost no public accounting of what those efforts have accomplished, much less of the ways they’ve failed, or of any toll they’ve taken.

This carefully choreographed arrangement is convenient for Mexico, as well. It allows that country’s government to assert that its police and armed forces do not take orders from the gringos. Meanwhile, the United States can claim credit when it helps Mexico capture a kingpin, but profess innocence when things go wrong.

Sergio Aguayo is a prominent Mexican human rights investigator at the Colegio de Mexico, which last year launched an independent probe into the Allende massacre. He told me: “The United States and its role remains an enigma. But the one thing that seems clear is that the government has its policies against organized crime, and pursues them without taking into account the impacts on Mexican society.” Aguayo said that may not be the intent, “but the affects are clear and inhumane.”

Certainly, the United States does not intend for massacres to happen. The DEA’s goal upon obtaining intelligence on the Zetas, part of an operation called Too Legit to Quit, was a good one: to bring an end to the cartel’s reign of terror. The agency had trained and vetted the members of the Mexican federal police unit with which it shared information about the intelligence. But the so-called Sensitive Investigative Unit (SIU) operates with a fundamental flaw that neither Mexico nor the United States has had the political will to fix: The unit’s Mexican supervisors are exempt from scrutiny.

“The unit is only as strong as its weakest link,” said a former senior DEA official who spent much of his career in Mexico. “If the supervisor isn’t vetted, then what’s the point of vetting anybody?”

But DEA agents I’ve spoken to said that Mexico would shut down the program before submitting its leaders to U.S. inspection. And the DEA believes having the vetted unit, even with the possibility of corruption, is better than not having it at all. Despite its weaknesses, they say, the unit has aided in important arrests.

Law enforcement sources close to the Allende case in United States blamed a Mexican supervisor in the vetted unit for the leak that triggered the Zetas’ attack. It wouldn’t be the first time. Over the two decades that I have been writing from and about Mexico, the SIU has been remade numerous times, either as part of a complete overhaul of the federal police, or because the unit had been infiltrated by drug traffickers. Or in some cases, the two went hand-in-hand.

Earlier this year, one of the unit’s supervisors, Ivan Reyes Arzate, turned himself in to authorities in Chicago who had charged him with leaking information to drug traffickers. Two of his predecessors at the unit, according to current and former DEA officials, were assassinated.

Expanded vetting couldn’t hurt. But it’s not foolproof, as the DEA has learned in partnerships with other national police forces. A vetted supervisor in Colombia, one DEA agent told me, was passing information to traffickers. “That information led to several cooperators being executed,” he said, “along with some of their family members.”

What the DEA could easily do, however, is establish firm and formal protocols for exchanging information with Mexico. Current and former DEA officials describe the system as loose and somewhat random, with agents independently making decisions — based on experience and relationships — about who to share information with both inside the DEA and among foreign counterparts.

This looseness is driven, in part, by shifting networks of corruption, some veteran agents said: Certain Mexican security forces might leak to a specific cartel, but reliably pursue another. They learned accordingly what to share with whom.

Another thing the agency could do is create mandatory accounting procedures that track the results of its operations. And finally, it could and should thoroughly investigate leaks, especially those with disastrous consequences, and report their findings to the Justice Department and Congress.

Yes, that may slow down a process that by its very nature requires speed — drug traffickers change their cellphone numbers every few weeks, precisely to avoid detection. But it could also save lives. There’s still no firm count of how many people were killed and kidnapped in and around Allende. A Mexican colleague and I found some 60 people whose deaths or disappearances have been linked to the Zetas’ siege by relatives, friends, court records, victims’ support groups and news accounts.

The DEA has been criticized for its role in other high-profile foreign drug operations gone awry, most recently in Honduras. If American authorities are going to operate in Mexican communities we should be prepared to treat the people who live in them as we would our own.