The United Nations human rights office has accused Mexico of ignoring fair principles of carrying out an investigation, saying authorities in the Latin American country have committed extensive rights abuses against suspects in a case related to the 2014 disappearance of 43 students.

Zeid Ra’ad al-Hussein, the UN High Commissioner for Human Rights, said in a report published on Thursday in Geneva that Mexican authorities had deliberately tortured dozens of people during the investigation into the fate of the students from a college in the town of Ayotzinapa who were allegedly abducted by corrupt police and then massacred by members of a drug cartel in the southwestern city of Iguala four years ago.

“The findings of the report point to a pattern of committing, tolerating and covering up torture in the investigation of the Ayotzinapa case,” said Zeid in the report.

The report, entitled “Double injustice - human rights violations in the investigation of the Ayotzinapa case,” is based on interviews, judicial files and medical records related to 63 out of 129 people detained in connection with the case.

The report said arbitrary detention and torture had been used in the investigation, adding that 51 people had been exposed to possible torture. The report said it had information that at least 34 of these individuals were definitely tortured, including one woman.

Zeid called on Mexican authorities to launch a full inquiry into the alleged rights abuses, adding that the case was a real test for officials to show their willingness and ability to tackle serious human rights violations.

“I urge the Mexican authorities to ensure that the search for truth and justice regarding Ayotzinapa continues, and also that those responsible for torture and other human rights violations committed during the investigation are held accountable,” said Zeid.

The report also called on authorities to remove any evidence in the Ayotzinapa case that was obtained under torture.

Mexico has yet to comment on the new UN report. Authorities announced on Monday that they had arrested a suspected drug gang member who was believed to be a key figure in the kidnapping of Ayotzinapa students.

Mexico has one of the highest homicide rates in the world. The ruling Institutional Revolutionary Party (PRI) has failed to tackle lawlessness and corruption. Polls suggest the party has a low chance for re-election in the upcoming presidential vote in July.