MEXICO CITY — An international committee of experts that spent years investigating the 2014 disappearance of 43 students in Mexico said on Monday that key evidence underlying the government’s explosive new truth commission report could not be verified as authentic.
The assessment came at a critical moment for President Andrés Manuel López Obrador’s administration, which vowed to get to the bottom of the disappearances but came under fire for conducting an investigation based in part on questionable evidence and bungling the prosecution of key suspects in the case.
In a news conference in Mexico City, the experts — who were appointed by the Inter-American Commission on Human Rights almost eight years ago — said they had uncovered numerous inconsistencies in evidence that the government presented as crucial to its new account of what had happened to the students, making it impossible to authenticate the material.
While the experts acknowledged the importance of the truth commission’s finding that various levels of government had participated in the students’ disappearance, they criticized the administration for presenting as fact evidence that turned out to be unsubstantiated.
The effort “to speed up the results of the case without taking the necessary investigative measures has resulted in a crisis,” said one of the international experts, Carlos Beristain, a Spanish human rights investigator. The government’s recent actions, Mr. Beristain added, “have put at risk the progress of the case and access to truth and justice.”
The experts’ findings followed a report last week from The New York Times in which the head of the government’s truth commission, Alejandro Encinas, said that “a very important percentage” of the screenshots underlying his report could not be authenticated. Mr. Encinas told The Times he still had confidence in his investigation, which he said was supported by other evidence.
In the wake of a wave of criticism in the Mexican news media after the article’s publication, Mr. López Obrador defended his administration’s handling of the case and called The Times unethical.
But the confirmation on Monday by widely respected investigators of problems with the screenshots is likely to increase pressure on the government as it scrambles to show progress on one of the most consequential investigations it has undertaken.
The 43 students vanished on a rainy night in September 2014, after commandeering a number of buses — a time-honored tradition — in order to take their peers to a demonstration in the capital commemorating a student massacre from decades ago. They were intercepted by gunmen and municipal police officers, who shot some and took them away.
Little is known about what exactly happened after that, though over the years evidence has emerged pointing to the involvement of a local drug-trafficking group, as well as federal security forces.
In August, the government published a report from a truth commission that said the disappearances had been a “crime of the state,” in which drug traffickers working with the federal police and the military had killed the students. Officials presented as key to that account a trove of 467 screenshots of messages purportedly sent by criminals and officials previously implicated in the crime.
The messages appeared to offer grisly new details about how and where drug traffickers had disposed of the students — and suggested that a high-ranking military officer had participated directly in the disappearance of six of them.
But the international experts said their forensic analysis of the screenshots had found several irregularities that made it impossible to confirm their authenticity, and concluded that “the analyzed messages cannot be considered digital evidence.”
The statement was a stinging rebuke to Mr. López Obrador’s administration, which had presented the report as a breakthrough in a case that for years has stagnated amid allegations of torture and apparent attempts by the previous administration to alter crime scenes.
The international experts said that there was solid evidence that the Mexican military had ties to the cartel that killed the students — and that members of the armed forces had known what was happening on the night they disappeared and did nothing to stop the massacre.
The international experts also said there was testimony from at least one person suggesting the army had held a group of the students captive.
The focus on the army, which investigators said had withheld evidence in the case, is likely to be a source of even greater tension for Mr. López Obrador, who has thrown his government’s full weight behind the military, putting it in charge of giant infrastructure projects and pushing to extend its role in national security tasks.
The committee also discussed how a parallel criminal investigation by the attorney general’s office into the disappearance has faltered in recent weeks.
After the attorney general sidelined the case’s special prosecutor, Omar Gómez Trejo, the government revoked more than a dozen arrest warrants against military officers, citing a lack of evidence in its own investigation. Mr. Gómez Trejo resigned and was replaced by a prosecutor who has little experience in the case.
On Monday, the international experts expressed concern that the resignation of the special prosecutor and other members of his team represented a “serious risk” for the case.
In another blow to the government’s investigation, the group of experts announced that two of its four members would be departing the group.
“Mexico has an opportunity to show that political will, together with the independence and consistency of the investigation, are decisive for the justice that the relatives are demanding,” Mr. Beristain said. “A case of forced disappearance is not closed until the right to truth has been delivered and the fate and whereabouts of the disappeared are determined.”