It takes a lot to stir up sustained national outrage in a country that has been ravaged by more than 200,000 murders and 30,000 disappearances in the last decade. Mired in a disastrous drug war, Mexico’s population has grown so accustomed to news of decapitations and bodies dissolved in acid that only the most nightmarish of crimes could provoke nationwide demonstrations or threaten to bring down the government.

That was precisely the kind of atrocity committed on 26 September 2014, a rainy Friday night that will never be forgotten. That evening, a group of trainee teachers from the Ayotzinapa college in the rural southern state of Guerrero arrived in the town of Iguala to commandeer buses to take them to Mexico City the following week. They intended to participate in an annual march to commemorate the massacre of scores of student demonstrators by the Mexican army in October 1968. Little did they know they would face a similar fate.

Local and federal police repeatedly attacked the students as they tried to leave Iguala aboard five buses. Officers shot dead three students and three bystanders, wounding dozens more. One student was found the next morning with his face flayed. Another remains comatose to this day. Forty-three students detained