An independent Mexican commission has found serious flaws in an investigation into the apparent massacre of 43 students last year, dealing a fresh blow to President Enrique Pena Nieto over a scandal that has blighted his administration.

The case became a symbol of impunity over disappearances, and plunged Pena Nieto into his deepest crisis, after the 43 trainee teachers were abducted in Iguala, and very likely murdered, by a drug gang working with corrupt police in southwest Mexico last September.

The report by the National Human Rights Commission (CNDH) said the attorney general's office, which has only identified the remains of one of the 43, still had not compiled basic information about the victims, who came from poor backgrounds. Nor had it properly investigated 11 suspects in the case, the CNDH found.

The findings add to a list of woes for Pena Nieto's government, which suffered major embarrassment this month when Mexico's most notorious drug lord, Joaquin "El Chapo" Guzman, escaped from a maximum security prison through a tunnel.

The mass disappearance in the city of Iguala "has proven the depth of barbarity ... the abandonment of the law and the neglect of justice" in Mexico, CNDH president Luis Raul Gonzalez said in a statement.

The attorney general's office said the drug gang had mistakenly identified the students, who belonged to a college with a radical left-wing tradition, as a threat and had them killed after clashes in Iguala the night of September 26.

Afterwards, their remains were incinerated, ground up and tossed in a river, the government determined. Tests identified the remains of one victim in December.

But the remaining families are still waiting for conclusive proof about the fate of their loved-ones.

A UN watchdog this year said the case was just one of thousands of "disappearances" carried out with the apparent complicity of security forces.

In the Iguala investigation, the CNDH denounced the use of arbitrary detentions and torture to obtain confessions in the case, and criticised the lack of testimony from people involved.

The commission also complained that prosecutors had only used statements from 36 soldiers, instead of interviewing everyone who had been in the area.

Iguala is home to a barracks, and questions have been raised about the army's failure to help.

Source: Agencies