A panel of international experts has accused Mexico's Government of undermining their probe into the fate of 43 trainee teachers apparently massacred in 2014, the most notorious human rights case in Mexico in recent years.

Key points: Independent panel says Mexican Government repeatedly blocked investigation

Independent panel says Mexican Government repeatedly blocked investigation Says Government's theory that students had been burned is scientifically impossible

Says Government's theory that students had been burned is scientifically impossible Government has opposed investigation's extension

The independent panel said the Government's stonewalling stopped them from reaching the truth as they wrap up their work and prepare to leave Mexico.

The Attorney-General's office, they said, did not let them re-interview detainees accused of the crime or obtain other information in a timely fashion. Prosecutors did not pursue investigative angles that the experts suggested.

"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.

The case has drawn international attention and stirred protests and outrage in Mexico, where violence has surged in a decade-long drug war. Lawlessness reigns in parts of the country and has tarnished President Enrique Pena Nieto's reputation.

Mr Pena Nieto thanked the experts via his official Twitter account. He said the Attorney-General's office would analyse their report.

Mexican marines search a mass grave site near Iguala, Mexico, finding 28 bodies.

"With openness, responsibility and adherence to the law, the [Attorney-General's office] will keep working so that there is justice," he said.

At a news conference on Sunday attended by more than 1,000 people, the experts cast doubt on aspects of the Government's version of events and said they had been repeatedly blocked in their efforts to obtain evidence from Mexican authorities.

As the experts finished their remarks, audience members yelled: "Don't leave!"

Mexico's Government said that corrupt police in late 2014 handed the student teachers in the south-western city of Iguala over to drug gang henchmen, who believed the trainees had been infiltrated by a rival gang. They then incinerated them at a garbage dump in Guerrero.

While the experts' probe showed the municipal police were mainly responsible for the disappearance of the students, they said the federal police should also be investigated.

One of the experts, Carlos Beristain, also said detainees in the case showed signs of torture.

The cartoon that appeared in Mexico's media showing its Attorney-General with a shovel, digging up skulls from a shallow grave.

The remains of just one of the 43 students has been identified from a charred bone fragment. The Government said it was found in the Rio San Juan, a river by the town of Cocula, near Iguala where the students disappeared.

The panel said that the Government's theory that the students had been burned was scientifically impossible given the heat needed to reduce human remains to ash.

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, referring to the college the trainee teachers attended.

The experts said they started to encounter stiff resistance from prosecutors in January.

Dozens of statements, most of which had been requested months earlier, were handed over about a week ago, when the experts were finishing up the report and could no longer analyse them.

The Reuters news agency reported last week that Mexico's army withheld crucial evidence from the experts, including photographs and video footage recorded as police clashed with the students, and that investigators had not been allowed to question soldiers on duty that night in the city where the students disappeared.

Protesters hold pictures of missing students, who police have confirmed were incinerated by drug gang members. ( Reuters )

Reuters