An international panel of experts accused Mexican government of undermining their probe into the fate of 43 trainee teachers missing since 2014.
The panel said Mexico's government is withholding key evidence in the case.
The experts accused the attorney general's office of not giving the re-interview detainees accused of the crime or obtain other information in a timely fashion.
The official leading the investigation for the attorney general's office said that the government had held numerous meetings with the experts and had fulfilled the majority of their hundreds of requests for information.
"The delays in obtaining evidence that could be used to figure out possible lines of investigation translates into a decision (to allow) impunity," the report by the experts, commissioned by the Inter-American Commission on Human Rights (IACHR), said.
On the night of September 26, 2014, 43 students from a leftist oriented college hijacked buses to go to a demonstration in the city of Iguala, their buses were shot at by the police.
According to the official report, the students were confused with rival gang members, drug cartel gunmen and corrupt policemen shot at their busses. The police then handed them over to the gang "Guerreros Unidos [United Warriors]" and the gang incinerated the students at a local dump.
But the families have never accepted the official report.
However, a September 2015 report, commissioned by the Inter-American Commission on Human Rights (IACHR) and conducted by respected investigators from Chile, Colombia, Guatemala and Spain, strenuously questioned the government's account, rejecting the central claim that the students were burned in the dump.
More than 1,000 people attended the news conference on Sunday, the experts cast doubt on aspects of the government's version of events.
They said in the report they had been repeatedly blocked in their efforts to obtain evidence from Mexican authorities.
"We feel that from January there was someone giving instructions to halt everything," one of the experts, Angela Buitrago told in an interview.
As the experts finished their remarks at the news conference, audience members yelled, "Don't leave!"
According to the experts’s probe, the municipal police is mainly responsible for the disappearance of the students, however they said the federal police should also be investigated.
The remains of only one of the missing students has been positively identified from a charred bone fragment. The government said the remains were found in the Rio San Juan river by the town of Cocula, near Iguala where the students disappeared.
The experts say that the government's theory the students had been burned is scientifically impossible given the heat needed to reduce human remains to ash. They also raised further questions in the report about the government's story of finding the bone fragment in the river.
According to the report, medical exams of the detainees claiming to be tortured are inadequate and do not meet international standards.
IACHR has said it will not renew the experts' term because the government was opposed to an extension.
"There seems to be no limit to the Mexican government's utter determination to sweep the Ayotzinapa tragedy under the carpet," Amnesty International's Erika Guevara-Rosa said in a statement.
Pena Nieto thanked the experts via his official Twitter account in Spanish. He said the attorney general's office would analyze their report.
"With openness, responsibility and adherence to the law, the attorney general's office will keep working so that there is justice," he said.
The attorney general's office offered the experts "full access to information", said deputy attorney general for human rights, Eber Betanzos.
Experts said, dozens of statements were handed about a week ago, as the experts were finishing the report and they couldn't analyze them.
Mexico's army allegedly withheld crucial evidence from the experts, including photographs and video footage recorded as police clashed with the students. It is alleged that, investigators have not been allowed to question soldiers on duty that night in the city where the students disappeared.