Ginger Thompson:

But unlike most places in Mexico that have been ravaged by the drug war, what happened in Allende didn’t have its origins in Mexico. It began in the United States, when the Drug Enforcement Administration scored an unexpected coup. An agent persuaded a high-level Zetas operative to hand over the trackable cellphone identification numbers for two of the cartel’s most wanted kingpins, Miguel Ángel Treviño and his ​brother Omar.

Then the DEA took a gamble. It shared the intelligence with a Mexican federal police unit that has long had problems with leaks — even though its members had been trained and vetted by the DEA. Almost immediately, the Treviños learned they’d been betrayed. The brothers set out to exact vengeance against the presumed snitches, their families and anyone remotely connected to them.
Their savagery in Allende was particularly surprising because the Treviños not only did business there — moving tens of millions of dollars in drugs and guns through the area each month — they’d also made it their home.
For years after the massacre, Mexican authorities made only desultory efforts to investigate. They erected a monument in Allende to honor the victims without fully determining their fates or punishing those responsible. American authorities eventually helped Mexico capture the Treviños but never acknowledged the devastating cost. In Allende, people suffered mostly in silence, too afraid to talk publicly.