Mexican prosecutors will target a former attorney general and his top aides in their investigation into the handling of a controversial probe into the disappearance five years ago of 43 student teachers, a government official said on Sunday.

The attorney general's office said on Saturday that prosecutors would hold to account those who oversaw the widely-panned probe into the abduction and apparent massacre of the trainee teachers by corrupt police working with a violent drug gang.

The scandal battered the reputation of then-president Enrique Pena Nieto. His successor, President Andres Manuel Lopez Obrador, has criticized how the case was handled and the release of suspects at the heart of the probe.

Deputy interior minister Alejandro Encinas, the official overseeing the reopening of the case, told a news conference 77 of 142 suspects detained over the September 26, 2014 disappearances in the southwestern city of Iguala had been released by courts.

The drug gang leader, Gildardo Lopez Astudillo, was recently released after a judge found officials had tortured him to get evidence. The UN human rights office said last year that authorities had probably tortured dozens of people in the case.

Encinas said the probe by prosecutors into the officials in charge of the case would focus "in particular" on Jesus Murillo, Tomas Zeron and Jose Aaron Perez Carro.

Murillo was the attorney general who presented the government's official account of what happened to the students, while Zeron headed the agency that led the investigation. Perez was in charge of a special office dedicated to the Iguala case.

Murillo and Zeron became lightning rods for public anger over the scandal, which helped undermine trust in the previous administration and pave the way for Lopez Obrador's landslide victory in last year's presidential election.

According to the official government account, the drug gang killed the students after mistaking them for members of a rival outfit. It said the killers then incinerated and ground up their remains, before dumping them in a nearby river.

It was there on October 29, 2014 that Zeron’s team said it found a bone fragment belonging to the only one of the 43 students whose remains have been definitively identified.

But a team of independent experts who picked holes in the account found that Zeron had been to where the fragment was discovered a day earlier with an alleged gang member, without notifying the man's lawyer or filing a report on his visit.

Zeron has denied any wrongdoing.