As demonstrators across Mexico take to the streets to protest the government’s involvement in the September 2014 disappearance of forty-three students in Iguala, Guerrero, a case bearing many of the same grim hallmarks is getting renewed attention.

In August 2010, the Zetas cartel abducted and killed seventy-two people pulled from buses traveling the highways near San Fernando, Tamaulipas, a town more than 1,000 kilometers northeast of Guerrero. In April of the next year, the remains of 193 people were discovered buried in dozens of mass graves in the same part of the state.

Seventeen San Fernando police officers were arrested and seven indicted in connection with the disappearances. But the details of the investigation remained under wraps until last month, when the Procuraduría General de la República, Mexico’s attorney general’s office, declassified a key document detailing the government’s findings. Released under Mexico’s transparency law at the request of the National Security Archive, a pro-transparency group based in Washington, DC, the three-page memo offers a small window into the secretive workings of the country’s justice system.

As in the Iguala case, federal authorities suspected that local officials helped organize the killings. We now know that those early suspicions were well grounded. Produced by the Subprocuraduría Especializada en Investigación de Delincuencia Organizada (SEIDO)—an investigative unit that focuses on organized crime—the memo describes a robust and routine pattern of narco-police collaboration in San Fernando. Captured Zetas told investigators that police acted as lookouts for the group, helped with “the interception of persons,” and turned a blind eye to their illegal activities.

“I know that police and transit officials in San Fernando help the Zetas organization,” testified Álvaro Alba Terrazas, one of the seventeen police officers detained. “Rather than take detainees to the Pentágano, which is to say the municipal jail, they would deliver them to the Zetas.” Terrazas also identified two other San Fernando cops on the Zetas’ payroll. The statement excerpted in the memo does not specify which prisoners were handed over to the Zetas or why.

One Zeta member captured in the months following the discovery of the gravesites told authorities that the cartel screened all buses passing through the region for passengers with ties to the rival Gulf Cartel. “Every day a bus would come, and every day we would pull the people off and investigate them,” said Edgar Huerta Montiel, also know as “El Wache.” Huerta said the Zetas would scan passengers’ cellphones and text messages for evidence linking them to the rival group. “Those that had nothing to do with it were freed, and those that did were killed.”

The revelation that San Fernando police worked hand-in-glove with the Zetas during the San Fernando massacre comes as little surprise to a Mexican public wearily accustomed to hearing accounts of state-sponsored violence and official corruption. But as Sergio Aguayo, one of Mexico’s leading human rights scholars, points out, there is a “big difference [between] the belief that something happened and having the information that shows it.” “In this case there is no longer any doubt,” Aguayo said on MVS Noticias. “The municipal police of San Fernando, Tamaulipas, were at the service of the Zetas.”

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The Zetas do not just co-opt government officials; they are also creatures of the state. Formerly secret US documents on the group, declassified under the Freedom of Information Act, together with media accounts and leaked diplomatic cables published by Wikileaks, lend credence to the firsthand accounts of police corruption described in the SEIDO memo, depicting the Zetas as a ruthless criminal organization with elite military training and an unmatched ability to compromise state and local security forces.

DEA and FBI intelligence reports trace the group’s roots to former members of the Mexican Army’s Airmobile Special Forces Group (GAFE), who trained at the US Army School of the Americas in Fort Benning, Georgia, before leaving the force more than a decade ago to run security operations for the Gulf Cartel. With additional training from the Kaibiles, a Guatemalan Special Forces unit responsible for grave atrocities in that country’s civil war, the Zetas ultimately broke away from the Gulf Cartel in 2010, sparking a round of gang warfare in Mexico almost unprecedented in its savagery.

Diplomats at the US Consulate in Monterrey, Nuevo León, had ringside seats as the Zetas and the Gulf Cartel battled during early 2010 for control of key trafficking routes in northeastern Mexico, engulfing towns in violence and penetrating deep into local police forces. During this time, Monterrey was a “safe-haven, source of revenue (mainly from extortion), and supply center for the Zetas,” according to a consulate report from February 26, 2010, which linked the former director general of state investigation of Nuevo León and other local and state police and government officials to the Zetas and other criminal groups.

The Zetas taunted their rivals, plastering the streets of Monterrey with “narcobanners” bearing the Zeta emblem claiming that the group was as popular as McDonald’s and cellphones. One of these, suspended from a statue only 100 meters from the Palace of Government, was hung “under state police observation,” according to the February 26 cable. In retaliation, the Gulf Cartel conducted a series of grenade attacks against local police as a “signal from the Gulf Cartel to the police to cease/desist their support of the Zetas.” Two days later, the consulate’s Emergency Action Committee, set up to report on the spiraling violence, predicted further Gulf Cartel attacks “against Zeta controlled police departments in the Monterrey area.”

In March, the Zetas launched an all-out offensive against Gulf Cartel targets in Nuevo León, turning March 2010 into what the US Embassy said was “one of the bloodiest months on record.” Narco-violence had “cut a swath across north-east Mexico,” according to the Embassy’s report. The cartels “operated fairly openly and with freedom of movement and operations” and “In many cases they operated with near total impunity in the face of compromised local security forces.” Nuevo León’s governor later suspended eighty-one state police officials after they admitted helping the Zetas put up dozens of roadblocks on roads headed north toward the US-Mexico border.

From 2009 into 2010, the Monterrey consulate catalogued numerous instances of official corruption and direct collaboration with criminal groups, including the fact that a staggering 165 of 1,000 state police officers had been “dismissed” for ties to the cartels. In one case, police “admitted to routinely serving as lookouts for drug trafficking organizations.” In what now seems an especially ominous warning of May 2010, the consulate noted that “Intense fighting between the Gulf Cartel and the Zetas has made travel chancy on roads north from Monterrey to the U.S. border.”

Three months later, the Zetas abducted seventy-five passengers from northbound buses in Tamaulipas, threatening to kill those who could not pay $2,000 in ransom and would not agree to work as Zeta assassins. In the end, seventy-two were murdered. One of the three survivors, an Ecuadorian who managed to escape his Zeta captors despite bullet wounds to his face and neck, later described the scene:

We entered the house in a row. Once inside, they bandaged our eyes.… we stood for about 20 minutes. I think they were waiting for nightfall. And then they placed us with our backs to the walls of the house.… They told us to lie face down, to be quiet, not to scream, because they were going to kill us.”

Grisly photos taken in the immediate aftermath of the killings show bound and bloodied corpses stacked up against the walls of the decaying warehouse where the slaughter took place.

Over the next year, hundreds of travelers fell into the hands of cartel thugs after being stopped at roadblocks along these highways. Many were never seen again. Most were undocumented and therefore difficult to identify. As Mexican authorities began to dig up the first of dozens of mass graves in San Fernando in April 2011, Mexican officials told US diplomats that “the vast majority of the remains appear to have been beaten to death.”

As the full extent of the slaughter began to take shape, US reports say, state and local officials in Tamaulipas immediately tried to minimize the extent of the bloodshed and hide the truth about what happened. After many of the corpses were transferred to other cities, Mexican officials “speaking off the record” told staffers at the US Consulate in Matamoros, the state capital, that the remains were being moved “to make the total number less obvious and thus less alarming.” The consulate said that Tamaulipas state officials “appear to be trying to downplay both the San Fernando discoveries and the state responsibility for them,” even while they were “fully cognizant of the hazards of highway travel in this area.”

Above all, the rise of the Zetas in Mexico stemmed from its ability to entice, co-opt and coerce government officials into compliance, trends noted nearly a decade ago in a 2005 FBI Intelligence Assessment. The Zetas had established control over Nuevo Laredo, the Mexican city directly across the border from Laredo, Texas, and a major point-of-entry for migrants headed to the United States. Calling the group “an emerging threat to the United States,” the FBI said the Zetas had “corrupted many Mexican public officials in the Nuevo Laredo area,” predicting, quite correctly, that “the government will likely achieve limited success at controlling their activities.”

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The release of the SEIDO memo has fueled hopes for greater transparency in government investigations of the many shocking massacre cases seen in Mexico over the last few years. But it didn’t happen easily. The attorney general’s office originally refused the National Security Archive’s request to declassify information on the arrests of the San Fernando police.

The SEIDO disclosure is largely thanks to the strategic legal efforts of a pair of Mexico-based non-governmental organizations that are trying to use Mexico’s access law to learn more about the killings in San Fernando and other human rights cases. While criminal investigations ordinarily proceed under extreme secrecy in Mexico, the law makes an exception in cases where the government has information that can shed light on “grave violations of human rights or humanitarian law.”

Citing this human rights exception, in December 2011 an advocacy group named Article 19 asked the attorney general’s office to provide a public version of its case file on the 2010 San Fernando massacre case. The Foundation for Justice (FJEDD), a legal aid organization, filed a similar case in April 2013, asking for files related to the 2010 massacre and the mass graves found, as well as a 2012 case where gangs left the dismembered torsos of forty-nine victims scattered along a highway in Cadereyta, Nuevo León.

In both instances, the attorney general’s office refused to divulge the files, citing exemptions in the access law allowing the government to withhold information related to ongoing investigations. The attorney general further argued that none of the massacres had been identified as human rights violations by a competent legal authority and that the agency was thus under no obligation to make the files public. Commissioners from Mexico’s Federal Institute for Access to Information (IFAI), an independent body that settles disputes over access issues, initially sided with the attorney general’s office and said it did not have the authority to compel disclosure without an authoritative finding of human rights crimes.

But a federal judge cast those arguments aside, ruling that the massacres indeed constituted human rights abuses, that IFAI did have the authority to act, and that the agency should order the attorney general’s office to release a public version of its investigative files. Most important, the court ruled that IFAI had violated the rights of the victims and their families in denying access to the information in the first place.

Citing international court decisions, United Nations declarations and the Mexican Constitution, the judge said transparency in human rights cases is key to “avoiding impunity and the repetition of such acts in the future.” The sensitive nature of investigatory files is precisely why it is so important to make them public, since they “affect society as a whole.” Mexico’s access law, he wrote,

foresaw an exception to withholding files on preliminary investigations in those extreme cases in which the crime being prosecuted is of such gravity that the public interest in maintaining the preliminary investigation files in secrecy is superseded by the interest of society as a whole in knowing of all the actions that are being brought to bear for the timely investigation, detention, judgment and sanctioning of those responsible.

The attorney general has appealed the decision. But in a positive sign for transparency, IFAI reversed its position on the FJEDD and A19 cases and dropped its opposition to disclosure of the investigative files. Soon thereafter, the oversight body ordered the attorney general to release the SEIDO memo, sending a strong signal that it will not stand in the way of future appeals for human rights information. In another promising sign, this month IFAI ordered the attorney general’s office to declassify investigative files on yet another recent human rights case: the alleged torture and extrajudicial execution of twenty-two members of an organized crime group by members of the Mexican Army in the Tlatlaya massacre.

The suspected involvement of police and other public servants in the disappearance of student activists at Iguala awakened Mexicans, and the world, to the continuing horrors and corrupting nature of the drug war in a way that migrant massacres far larger in number never have. Indeed, thousands of similar cases—if smaller in number—have failed to produce the kind of outrage seen after the forty-three vanished in September. In a tragic but ironic twist to the Iguala story, investigators digging for evidence in that case have uncovered the remains of at least thirty-nine other individuals whose killings were not related to the Iguala disappearances. Mexico is only now beginning to understand the scale of the violence that has plagued the country over the last decade.

Still, demands for justice are ringing louder than ever before, and because of this—and the tiny new window on the San Fernando case offered by the declassified PGR memo—there is hope that Iguala will be different, that justice will provide some measure of closure for the families of the victims, and that a new commitment to transparency will help Mexico avoid “impunity and the repetition of such acts in the future.”

National Security Archive researcher Jesse Franzblau contributed reporting to this piece.