International experts reviewing one of Mexico's most notorious crimes have accused parts of the Mexican government and its sympathizers of trying to block advances in the investigation and orchestrating a smear campaign against them.

The five members of an experts' committee convened by the Inter-American Commission on Human Rights (IACHR) to keep an eye on the government's investigation into the kidnapping of 43 students in September 2014, made the allegations at a press conference on Wednesday.

"There are sectors who want to block the investigation, center all the discussion on the garbage dump, break up the file, ignore different lines of inquiry, and even try to close the case," the experts said in a statement that they read out at the conference. "We have alerted Mexico's highest authorities of the necessary structural changes, including a change of mentalities."

The press conference also included the accusation of a smear campaign at a time when an onslaught of unflattering information about the IACHR experts has spilled into the Mexican media. A battery of stories have questioned the experts' competence, and even alleged inappropriate acts, such as mismanaging public money.

The rising tension comes within a broader souring of relations between the government and human rights organizations.

The United Nations High Commission on Human rights issued a statement on Wednesday condemning "a stigmatization campaign that attempts to undermine those that work as promoters of fundamental freedoms in the country." The Commission demanded that the Mexican government "counter" the attacks.

The government's particular issues with the Inter-American Commission's experts also come as it appears to be doubling down on its original investigation into the attacks on the students from the radical Ayotzinapa teacher training college as they commandeered buses in the city of Iguala.

The disappearance of the students shattered President Enrique Peña Nieto's international reputation as a reformer and sent his popularity at home plunging.

His image was also damaged by the widespread skepticism that greeted the government's initial conclusions, revealed in November 2014, that the students' bodies were burned in a garbage dump during an all-night inferno. Then attorney general Jésus Murillo Karam called it "the historical truth."

The government originally invited the IACHR investigators to work in the country in a bid to prove the trustworthiness of its investigation. But this became a major problem when the IACHR experts released a report in September 2015 that dismissed the garbage dump funeral pyre hypothesis as scientifically impossible because of the lack of evidence of a major fire at the site. They subsequently reported rain falling on the night in question and satellite images showing no fires in Guerrero state, which unfolds to the south of Mexico City and includes the sites of the attack and the dump.

The government initially responded by promising a new technical investigation by a six-person team of specialists to review evidence on the fire theory. The IACHR experts said this was unnecessary and that new lines of investigation should dominate. But they also applauded the government's subsequent promise to also relaunch the wider investigation with a new team, as well as efforts to improve relations with the relatives of the missing students.

Then, this year, the advances were reversed.

Peña Nieto appeared to signal a split with a February trip to Iguala. He centered his first visit to the city where the attacks happened on lauding the military, which could seem like a snub to the failed efforts by the international experts to question soldiers from the Iguala barracks about what happened on the night the students disappeared.

In the meantime newspapers with pro-government positions have begun to regularly run headlines questioning the IACHR experts' activities and investigation.

Watch the VICE News documentary The Missing 43: Mexico's Disappeared Students

Well-known anti-crime crusader and sporting goods mogul, Alejandro Martí, whose teenage son was kidnapped and killed in 2008, has questioned the experts' investigation.

Another anti-crime figure José Antonio Ortega Sánchez filed a criminal complaint for fraud against IACHR executive secretary Emilio Álvarez Icaza. A file was initially opened, though it was then quietly closed this week.

Álvarez Icaza told the newspaper El Universal that he had no knowledge of a similar act anywhere in Latin America — even in countries such as Venezuela under the late president Hugo Chávez or in Peru under strongman Alberto Fujimori.

"Mexico is experiencing a return to authoritarianism," he warned.

Relations between IACHR experts and the Mexican government turned especially tense last Friday, when the attorney general's office called a press conference — without advising the experts — in which one of the fire specialists said that there was a controlled burn and 17 adult remains found at the garbage dump. None of the remains have been identified.

The IACHR experts say the information provided on Friday was preliminary, not conclusive and they didn't know details of what the fire specialist had determined. The group also says that the airing of the information violated an agreement to work collaboratively and only inform the media after coming to a consensus. Such breaches of trust, they add, mean they can no longer participate in the committee of fire specialists.

For its part the Interior Ministry has repeatedly stated that the experts work will end as scheduled April 30 and its mandate will not be renewed. (The IACHR says its experts can stay longer if they want.)

The Peña Nieto administration has repeatedly denied any suggestions of impropriety in the area of human rights.

On Tuesday the president celebrated a 74 percent fall in the number of cases resulting in recommendations for action from the National Human Rights Commission since he took office in December 2012. He also said Mexico should be proud that it has enjoyed eight decades of political stability.

"Few nations can say the same," he said.

Most of those decades were spent under the control of the Institutional Revolutionary Party, or PRI, that governed from 1929 until 2000 as well as from 2012 until now. One historian sees a return to the PRI tactics of old in its strategy in the face of international criticism on human rights.

"Instead of confronting them and saying, 'You're not allowed to investigate,' the government tactic has been to discredit them here, saying that these are inventions, that it's inept, that it doesn't work,'" said Ilán Semo, political historian at the Iberoamerican University. "This has been the government tactic to undercut the Ayotzinapa investigation. It's not really prohibiting the commission or threatening it."

Others see the president following an old PRI belief that the country will always, eventually, fall in line with the official version, as long as it is repeated enough.

"It's the old PRI and they have a mindset, where they expect even the incredible to be taken at face value by society at large, especially when prodded by the elites," said Federico Estévez, political science professor at the Autonomous Technological Institute of Mexico.

"The elites decide, then it's beaten down," Estevéz said. "If you pressure enough, it eventually goes down."