One of Mexico's top crime fighters is under investigation over a key incident in the probe into the apparent massacre of 43 trainee teachers in 2014, the attorney general's office says.
The probe into officials including Tomas Zeron, head of the attorney general's criminal investigation agency, was triggered on Thursday by a report by a panel of experts questioning the circumstances in which a charred bone fragment was discovered in a river.
The fragment belonged to the only one of the 43 whose remains have been definitively identified, evidence which has sustained the government's version the teachers were murdered by a drug gang, incinerated and dumped into the river.
Zeron came under the spotlight on Sunday when the team of international experts presented video and photographs suggesting he was at the site where the bone was found a day before its official discovery, but made no report of his visit.
The attorney general's office said in a statement an internal disciplinary body would "investigate the actions of the public servants of the institution in these events."
Zeron's office did not immediately respond to a request for comment. But responding to the panel's disclosures on Wednesday, Zeron said the visit was legal and that he had been accompanied by representatives of the UN human rights office. He did not explain why the trip was not officially documented.
Mexico's UN human rights office said it had not accompanied Zeron, saying in a statement it "finds it strange that its presence and role were mentioned in activities of the attorney general's office it never took part in."
The panel of experts has picked holes in the government's official account of how the 43 students disappeared in the southwestern city of Iguala in late September 2014.
Although many questions remain unanswered, the government said the panel's participation in the case would finish this month, and there have been a number of claims and counterclaims between the two sides over the investigation.
Zeron is seen as close to President Enrique Pena Nieto and had a lead role in the 2014 capture of Mexico's most wanted drug lord, Joaquin "El Chapo" Guzman. Guzman escaped from prison the following year, before being recaptured in January.