Massacres and mass graves are rarely a surprise in Mexico anymore. The nation’s drug gangs have periodically used them as a public intimidation tactic or to one-up their rivals with escalating displays of large-scale savagery.

But the discovery Saturday of 28 bodies in a charred thicket on the outskirts of Iguala, a town 125 miles south of Mexico City, is a different kind of horror. The corpses turned up about a week after 43 college students vanished in the town while protesting new education laws. Some of the missing were last seen in the custody of local police.

Mexican investigators caution that it may take up to two months to identify the dead because they are burned beyond recognition. In a region of warring crime gangs that one resident called “a land of the wicked,” it is possible — and would be perhaps even more appalling — that the bodies are linked to an unrelated mass killing.

But two suspects in custody have confessed to helping kill 17 of the students at the behest of local police, according to Guerrero state prosecutor Iñaky Blanco.

If that is true, the Iguala deaths would mark a terrifying new low in the convergence of political violence and mafia-style barbarity, human rights observers say, in which civilian protesters who have nothing to do with the drug trade are butchered by cartel henchmen as a favor to local authorities.

Authorities uncovered a mass grave in an area where dozens of people were missing after clashes with police and armed men last week. (Reuters)

“The fact that the police would think they could get away with killing so many students and burning their bodies is indicative of how bad the situation in Mexico has become,” said José Miguel Vivanco, the Latin America director at Human Rights Watch. He said President Enrique Peña Nieto has been more concerned with polishing Mexico’s public image than taking serious steps to address gross violations of basic rights.

“By repeatedly failing to investigate disappearances and other abuses, this administration and its predecessor have created a climate in which impunity is the norm even for the most atrocious crimes,” Vivanco said.

In a statement Monday, Peña Nieto promised that the massacre would not drift down Mexico’s black hole of unsolved crimes. “Mexican society, and the victims’ families, rightfully demand to find out what happened, that justice be done, and that those responsible are caught and no impunity be allowed,” he said.

More than 22,000 people have disappeared in Mexico since December 2006, when then-President Felipe Calderón launched a military-led assault on the drug gangs. Often the missing have no ties to the smuggling business.

But it is unusual for this type of mass killing to be the direct result of an act of political protest.

At least 30 other suspects have been taken into custody in Iguala, including 22 local police officers and several alleged members of the Guerreros Unidos criminal organization.

Mexican authorities say they are still searching for the mayor and the town’s top police official, who have possibly fled.

Iguala is of strategic value to trafficking groups because it is located between the capital and the lowland Tierra Caliente coastal region notorious for methamphetamine labs and rural lawlessness.

Small towns in southern Mexico such as Iguala also have a long history of radical and sometimes violent campus activism, including the temporary commandeering or “borrowing” of buses to transport protesters. The disappearance of the 43 students on Sept. 26 followed a day of chaotic and confusing events.

According to state prosecutors and local news accounts, about 80 students from the local Ayotzinapa teachers college spent the day demonstrating in town and trying to raise money for their cause. Then they boarded three buses at the town’s central station, forcing their way on, according to some versions.

Mexican investigators say they don’t know what specifically triggered the violence that followed, but at some point, local police apparently responded to the students’ actions by enlisting members of the Guerreros Unidos gang, led by a fearsome drug boss nicknamed “El Chucky.”

When the buses drove off, gunmen opened fire, forcing them to stop. Some of the students ran, but about 20 were taken into custody. Three were later found dead, including one whose face was skinned off and eyes gouged out.

Several hours later, masked gunmen shot at students who had returned to the scene and were giving interviews to reporters. Stray bullets killed three bystanders: a bus driver, a 15-year-old and a woman in a taxi.

Over the weekend, students joined family members of the missing in protests that blocked the main highway between Mexico City and Acapulco. In the Guerrero state capital of Chilpancingo, a student group flipped over a car outside the governor’s residence and threw homemade firebombs.

Some of the parents who viewed the photos of the charred bodies insisted that they were not the missing students, believing authorities are keeping them in custody.

“We don’t know where they are, or if they’re hungry, or if they’ve been beaten,” said Maria Abraham, whose nephew is among the 43, while speaking to television crews along the highway.

Mistrust of the government runs so deep in the area that investigators have brought in forensic experts from Argentina to assist with DNA testing.