The disappearance of 43 students after an attack by gang-affiliated police has triggered a wave of outrage in Mexico over a shocking example of collusion between authorities and organised crime.
Mexicans have been exposed to untold horrors in a drug war that has left 80,000 people dead since 2006, but the latest discovery of mass graves linked to the case has caused an uproar.
Tens of thousands of people held protests across the country to demand the return of the 43 students, who were studying to become teachers in the violence-wracked southern state of Guerrero.
The students vanished after police linked to the Guerreros Unidos gang attacked buses they had seized in the city of Iguala, 200 kilometers (125 miles) south of Mexico City.
A mass grave containing 28 bodies was found outside the city on Saturday and authorities announced on Thursday the discovery of four more pits. Suspects told authorities that students were dumped in the unmarked graves.
While officials stress that it will take time to identify the victims, the grim discoveries have raised fears about the fate of the 43 students.
Prosecutors want to question Iguala's Mayor Jose Luis Abarca, his wife and the city police chief, but all three have apparently gone into hiding.
Pain, anger and shame are three recurring words in Mexican media to sum up the national sentiment -- pain for the missing and their families, anger against the authorities and shame for Mexico, which has been urged by the United Nations, the United States and human rights groups to get to the bottom of the crime.
Fears of a massacre has tarnished President Enrique Pena Nieto's pledge to reduce violence in Mexico and ensure that human rights are respected.
- 'Impunity reigns in Mexico' -
Pena Nieto acknowledged the growing national anger over the "inhuman" case on Thursday, saying it had caused "dismay not only among Mexicans, but also in many parts of the world that expressed their repudiation and outrage."
Before the Iguala scandal, Pena Nieto was already wrestling with the fallout of allegations that soldiers executed more than 20 drug suspects in the town of Tlatlaya, outside Mexico City, in June.
Three soldiers have now been charged with the murder of eight of those suspects.
Pena Nieto has highlighted a drop in murders in Mexico in the past two years as well as the capture of several top drug lords, the latest being Juarez cartel boss Vicente Carrillo Fuentes on Thursday.
"It's useless to say that homicides are down if certain scandals give a different impression," wrote Reforma newspaper columnist Sergio Sarmento.
Alejandra Cullen, a political analyst, said the Iguala case "shows the takeover of power by organised crime."
"A line has been crossed," Cullen said.
The Iguala tragedy has caused shame within the leftist Democratic Revolution Party (PRD), whose leaders went to Iguala on Monday to apologize to Guerrero's citizens for allowing Abarca to represent the PRD in the 2012 elections.
The anger is so sharp that protesters threw projectiles at the PRD's historic leader, Cuauhtemoc Cardenas, when he attended a rally for the students in Mexico City on Wednesday. Some shouted "assassin" at him.
Pena Nieto vowed justice in the case, a pledge tough to fulfill in a country where most murders go unpunished.
Cullen said impunity exists in Mexico due to "the partnership between criminals and authorities."
Alejandro Hope, a security analyst and former intelligence officer, wrote in El Universal newspaper that the Iguala attacks were possible because "impunity reigns in Mexico."