Officially, cartel members killed the students and incinerated their remains. But exactly what happened to them has been hotly disputed.

In August, a government-backed truth commission branded the tragedy a "state crime", and said the military shared responsibility -- either directly or through negligence.

There was "an internal decision not to provide more information" about the incident, Carlos Beristain, a member of the Interdisciplinary Group of Independent Experts (GIEI), told reporters Friday.

He said the commission's experts had seen a document from the General Staff of the Mexican Defense Secretariat with instructions "to give an agreed response".

"That for us is not acceptable," Beristain said, adding that witnesses had also informed the commission that intelligence documents were dispersed to various locations to hide them.

The GIEI, a body created in 2015 by an agreement between the Mexican government and the Inter-American Commission on Human Rights (IACHR), has "renewed" elements of the investigation, said Colombian lawyer Angela Buitrago, also a member of the truth panel.

This means the prosecutor's office could reactivate arrest warrants -- which were canceled in September -- against around 20 military personnel involved, Buitrago said.

She added that Mexican President Andres Manuel Lopez Obrador has "directly requested a report".

Initially, investigators said the students, who were en route to a demonstration in Mexico City, were detained by corrupt police and handed over to a drug cartel that mistook them for members of a rival gang.

An official report presented by the government in 2015 concluded that cartel members had killed the students and burned the remains at a garbage dump.

Only three victims have been identified.

The initial findings, which did not attribute responsibility to members of the armed forces, were rejected by relatives and independent experts.

Mexican authorities last month announced the arrest of nine Guerrero state police officers suspected of involvement in the disappearance, while the United States deported a former police officer also implicated in the disappearance in January.