Two weeks after 43 students disappeared in rural southern Mexico after clashes with police, authorities say the suspects have led them to more mass graves.

Dozens of anxious parents have gathered at a teaching college that was supposed to be their sons’ escape from life as subsistence farmers, waiting for word on the fate of their sons and holding prayer sessions in a makeshift shelter on the school’s covered courtyard.

“They took him away alive, and that’s the way I want him back,” said Macedonia Torres Romero, whose son José Luis is among the disappeared.

But that seems ever more unlikely as time passes and new graves are found.

The attorney general, Jesus Murillo Karam, announced the arrest of four more people on Thursday, taking the total number of those detainedto 34, including 26 police officers. The suspects led investigators to four new burial pits near the southern city of Iguala, where authorities unearthed 28 badly burned bodies last weekend.

Forensic tests are being carried out to determine whether any are the missing students.

Prosecutors attribute the disappearances to police, who killed six people and wounded at least 25 in two separate attacks in the city of Iguala, after which officers rounded up some students and drove off with them. Police are believed to have turned over the students to a local drug gang that apparently had ties to the family of Iguala’s mayor, Jose Luis Abarca.

The case has outraged Mexicans in a country where abuse of authority is common in remote areas.

That 43 young men went missing at the hands of the state has drawn calls from around the world for justice, including the US state department and the Organisation of American States, where the secretary general, José Miguel Insulza, said all of Latin America was grieving.

The Guerrero state prosecutor, Inaky Blanco, said he was asking the state congress to strip the mayor of the political immunity elected officials have under Mexican law. Abarca, who authorities say is on the run, may also face charges for not intervening to stop the attacks.

The students and their families come mostly from the remote mountains of the southern state of Guerrero, where they live in poverty under the thumb of corrupt governments, drug traffickers or armed vigilante groups that have sprung up in reaction to the region’s lawlessness.