By Anahi Rama and Lizbeth Diaz

MEXICO CITY (Reuters) - A panel of international experts on Sunday 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.

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.

At a 2-1/2-hour news conference on Sunday attended by more than 1,000 people, 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 Reuters in an interview on Sunday night.

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

Mexico's government says that corrupt police in late 2014 handed the student teachers in the southwestern 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 the southwestern Mexican state of Guerrero.

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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.

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 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, and the experts raised further questions in the report about the government's story of finding the bone fragment in the river.

One of the experts, Carlos Beristain, also said detainees in the case showed signs of torture. The experts´ report notes that medical exams of detainees who said they were tortured were inadequate and did not meet international standards.

IACHR has said it will not renew the experts' term because the government was opposed to an extension. Mexico's government authorized the group´s investigation, vowing to cooperate fully, but at times actively blocked them.

"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.

Pena Nieto thanked the experts via his official Twitter account. 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.

In a statement, the official who is 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 attorney general's office offered the experts "full access to information", said Eber Betanzos, deputy attorney general for human rights.

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 analyze them.

Reuters 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 have not been allowed to question soldiers on duty that night in the city where the students disappeared.

(Reporting by Anahi Rama and Lizbeth Diaz; Additional reporting by Luis Rojas; Writing by Joanna Zuckerman Bernstein; Editing by Simon Gardner and Cynthia Osterman)