Mexico’s president has promised to investigate the botched investigation into the abduction and disappearance of 43 teacher trainees in 2014 – one of the country’s most notorious crimes and an atrocity the then government tried to downplay as a minor local matter.

Andrés Manuel López Obrador made the announcement on Wednesday following the release of an alleged leader of the local drug gang believed to have killed the missing students who were abducted by corrupt police officers in the town of Iguala.

Experts say the release of Gildardo López Astudillo sets a precedent that could lead to the release of 50 more suspects in a case that has come to symbolize Mexico’s human rights crisis.

Another 142 suspects have already been set free, said Alejandro Encinas, deputy interior secretary. He blamed authorities’ fabrication of evidence, use of torture and a general cover-up for destroying a case where real evidence existed to prosecute those responsible.

“It’s a very serious justice issue and because of that we’re going to formally file a complaint with the attorney general’s office and the judiciary in this case,” said López Obrador.

Authorities say the students from the Ayotzinapa teacher training college were abducted by police and handed over to the drug gang Guerreros Unidos. López Astudillo allegedly headed the gang in Iguala.

Jesús Murillo Karam, attorney general under former president Enrique Peña Nieto, infamously announced that authorities had established the “historic truth” of the students’ disappearance. He said that the students were killed and their bodies incinerated at a garbage dump.

But independent experts said there was no evidence to support that conclusion, and investigators have definitively identified the remains of only one of the 43 students.

“The poorly named ‘historic truth’ was built with a foundation of cover-up, fabrication of evidence and torture to the benefit of the perpetrators and against the victims’ rights,” Encinas said. “The historic truth collides with reality.”

Encinas made clear Murillo Karam and other top officials involved in the case should be included in the investigation.

“What we have now are a series of trials, which are falling apart,” said María Luisa Aguilar, international affairs director for the Miguel Agustín Pro Juárez Human Rights Centre, which has worked with the students’ families.

“The impression the families have is that the investigators tried to shelve the investigation,” she added. “They didn’t do a proper investigation into what happened or the students’ whereabouts – which is what the families care about most.”

The UN’s human rights office said in a report last year that Mexican authorities had probably tortured dozens of people during the investigation.

Peña Nieto’s popularity suffered lasting damage from the scandal over the disappearances, and his government was heavily criticized for its handling of the investigation.