A panel of international experts investigating the disappearance of 43 Mexican trainee teachers in 2014 says the government of President Enrique Pena Neto has hampered its inquiries.
In its scathing final report, the experts also dismissed the conclusions of the official inquiry.
They said officials failed to pursue the investigative lines they suggested.
The case provoked outrage in Mexico, leading to street protests against perceived impunity.
"The delays in obtaining evidence that could be used to figure out possible lines of investigation translates into a decision to (allow) impunity," said the Interdisciplinary Group of Independent Experts (GIEI) in its report.
The panel was commissioned by the Inter-American Commission on Human Rights (IACHR).
The trainee teachers went missing after taking part in a protest in the south-western city of Iguala, in Guerrero state, in September 2014.
Mexican prosecutors said they were detained by corrupt policemen under the orders of the mayor of Iguala, Jose Luis Abarca, and handed over to a local criminal gang (Guerreros Unidos), who killed the 43 men and burned their bodies in a local landfill site.
Relatives have always rejected this version, saying the government was trying to cover up the involvement of senior politicians and army officers in the killings, foreign media reports.