Mexico's government has silently implemented a law blocking public access to archival documents related to the country's Guerra Sucia, or Dirty War, which left thousands of political dissidents, students, and activists dead or disappeared in the 1960s, 70s, and early 80s.

The Dirty War was a period of low-intensity internal conflict between the ruling Institutionalized Revolutionary Party (PRI) and leftist guerrilla movements, which surged during the PRI's more than 70 years in power.

The most notable result of this civilian insurgency was the 1968 Tlatelolco student massacre. The Mexican military opened fire on protesting students, teachers, and passersby, killing hundreds — the official death toll was never definitively determined and remains unknown.

The Dirty War archive was sealed this year as a delayed result of the 2012 Federal Law of Archives, which reclassified these historic documents as "confidential." The controversial move further obfuscates the details that surround a dark period in Mexican history marked by unofficial military operations aimed at wiping out insurgent guerrillas and their leaders, which continue to be muddied.

The historic document collection that was open for public viewing at Mexico's National Archives for more than a decade is now off limits to the public.

After several days of consultation, officials at the Secretary of the Interior's office told VICE News they were unsure when exactly the order went into effect, but they believed it was sometime in mid-March. Local news reports in Mexico made the reclassification of the documents public knowledge earlier this month.

Now, in order to peruse the archives, you must submit a request in writing and — if your request is approved — the information you obtain can be redacted, summarized, or otherwise altered. The policy change comes despite the fact that the Mexican constitution guarantees complete transparency of public information.

The archives are housed at the Lecumberri Palace, a fortress-like structure in Mexico City opened by President Porfirio Díazin as a penitentiary in 1900. The former prison, known historically as "the Black Palace," has housed many of Mexico's most infamous criminals. Author William S. Burroughs famously served a stint behind its doors after shooting his wife in 1951 in Mexico City during a drunken game of "William Tell."

Twenty five years later, the jail closed its doors and became home to one of the earliest public records collections in the Americas.

Entering the National Archives building is an easy task. After presenting identification, you are allowed in with a pen and paper. If, however, you are interested in reviewing documents pertaining to the Dirty War, finding what you need has become quite complicated.

'I think they send you what they think is right for your research.'

As soon as you express interest in the records, the typically friendly staff becomes visibly nervous and tells you it's impossible. Then they offer you a catalog of unrelated records to peruse.

"The documents can't be consulted like before," Maricela Gavia, one of the government workers responsible for the reference section of the archives, told VICE News. "First you have to fill out a form with the IFAI [the Federal Institute for Access to Information], which is then sent to Mexico's intelligence agency, or CISEN, so they can give the final approval."

"I think they send you what they think is right for your research," she added.

The documents related to the abuses that occurred during the Dirty War were first opened in 2001 for public consultation by former President Vicente Fox, whose election the previous year as a candidate with the National Action Party, or PAN, interrupted more than 70 years of single-party rule in Mexico.

The Fox administration ordered the opening of the files as a way to cast light on a period of 20th century Mexican history that had been largely silenced and shrouded by the PRI during its administrative stronghold. The hope was that the unsealing would help achieve justice for families of victims persecuted because of their participation in the social and political movements during the PRI era.

After a 12-year interruption after more than half a century in power, the PRI once again regained executive control in 2012 with the election of President Enrique Peña Nieto. Since gaining office, Peña Nieto has vowed to enact transparency while simultaneously being accused of attempting to influence an investigation against him over conflict of interest allegations.

"This is just wrong," Maria Herrerías, a professor from the National Autonomous University of Mexico who specializes in Mexican guerrilla movements, told VICE News. "It is a total violation of our right to have free access to information."

According to Herrerías, before the federal archival law went into effect, documents could only be kept from public view for a maximum of 10 years. But under the new system, these documents will now remain sealed for 30 years, and if they contain personal information that number climbs to 70 years. But, as Herrerías noted, documents that evidence human rights violations should not be considered personal.

"Those archives are part of our national memory," she said. "They are matters of public interest, not something that affects a small group of people."

The Dirty War left an estimated 2,000 people disappeared and a largely disputed number of political dissenters dead. Most of the victims of this period of political oppression were leftist guerrillas, college students, and activists.

In 2006, the Mexican government issued a report on the Dirty War, but left out major historical points, claiming they were biased against the Mexican army.

The families of many Dirty War victims are still awaiting justice, and the repercussions of this murky period in Mexican history are evident in the human rights abuses that have persisted in the country.

Prominent guerrilla leader Lucio Cabañas is one of the most noted victims. A teacher turned revolutionary from the state of Guerrero, Cabañas began a rural movement aimed at protecting the poor from exploitation. He and many of his followers were violently executed or disappeared during the Dirty War, and even now, three decades later, most of these cases remain unsettled.

Cabañas is one of the best-known graduates of the Ayotzinapa Normal School, the rural teacher's college in Guerrero now infamous for the brutal police attacks that occurred in September 2014, leaving six people dead and 43 students disappeared. Some of the Ayotzinapa students are direct descendants of Cabañas and his followers, and some Mexicans view the repression they have suffered as a carryover from the past decades of state-orchestrated violence.

Human rights advocates have condemned the closure of the archives, describing it as arbitrary and secretive. They fear it could impact other important historic documents, such those related to the disappearances that occurred during the Felipe Calderón administration — a figure that human rights organizations estimate to be more than 26,569.

Last October, the Guerrero State Truth Commission published a report on human rights violations that occurred during the years of conflict, and a partial list of victims. The commission concluded that there had been systematic repression perpetrated by local governments with huge resources to neutralize those they deemed subversive.

The Association of Relatives of the Detained and Disappeared (AFADEM) criticized the report and said that the government accomplishes nothing with these publications.

"There has been no truth, and no justice," AFADEM head Tita Radilla said at the time. "None of these cases have been resolved, and none of the institutions are interested in resolving these events from the past."

Herrerías said that keeping the archives open for consultation should be considered a matter of public interest, particularly for families of the missing.