The first group of mass graves excavated on the outskirts of a town in Mexico’s Guerrero state does not contain the bodies of any of the 43 students who went missing last month, Mexico’s attorney general said. The whereabouts of the students, who disappeared after a confrontation with police in late September, have been the subject of mass protests in the state.

Authorities have been conducting DNA tests on the 28 badly burned bodies found in the graves since discovering them earlier this month. Attorney General Jesus Murillo Karam said tests were continuing on four additional graves uncovered last week.

State governor Angel Aguirre said some of the bodies could have been earlier victims of criminal gangs, and some of them seemed to have been buried “months ago.”

Murillo Karam, speaking at a news conference Tuesday, added that 14 additional police officers were arrested over the disappearances. So far, 50 people, mostly police, have been arrested for their roles in the incident, and prosecutors accuse police of colluding with a local gang known as Guerreros Unidos. The police chief and mayor of Iguala, the town where the confrontation between police and students occurred, have also been accused of ties with the gang. Both are currently in hiding.

On Tuesday, the leader of Guerreros Unidos, Benjamin Mondragon, committed suicide by shooting himself when confronted by security forces in the neighboring state of Morelos. Federal authorities said it wasn’t clear whether Mondragon was involved with the student disappearances.

Hundreds of protesters, many from the teachers’ college that the missing students had attended, took to the streets in Guerrero’s capital of Chilpancingo this week over the disappearances. The protests turned chaotic on Tuesday when demonstrators overtook state government buildings, ransacked offices and burned parts of the complex. On Tuesday, they took over bank branches and a local radio station and vowed to keep up the pressure on Guerrero’s government until the missing students were found.