MEXICO CITY: Mexican authorities said Friday they have arrested the top leader of a drug cartel linked to the disappearance of dozens of college students in the city of Iguala last month.

Sidronio Casarrubias Salgado was detained by federal police Thursday on a highway outside Mexico City, Attorney General Jesus Murillo Karam said.

Murillo Karam identified Casarrubias as the “supreme leader” of Guerreos Unidos, the gang he says colluded with local police in the abduction of 43 students three weeks ago after a violent protest in Iguala, in Guerrero state. The students have not been heard from since. Investigators believe they may have been killed and dumped in several mass graves discovered recently in the region.

Early this week, a different man identified by authorities as the leader of Guerrero Unidos killed himself after a standoff with federal police. Neither that man, Benjamin Mondragon Pereda, nor Casarrubias is alleged to have ordered the detention of the students, according to the attorney general’s office.

At a news conference Friday night Murillo Karam and another official in his office did not explain why Mondragon had first been identified as the leader of the gang. They also suggested that neither was responsible for ordering the abduction of the students, although they did not elaborate.

Protests over the government’s handling of the case have been mounting across Mexico in recent days, with demonstrators accusing federal officials of ineptitude for failing to determine what happened to the students.

On Friday, thousands of students and teachers marched in Acapulco, a tourist destination in Guerrero, demanding that the students be found alive. It was a peaceful protest compared to a demonstration this week in Chilpancingo, the capital city of Guerrero, where protesters set fires at a government building.

The protests have stretched from the southern state of Oaxaca, where angry students took over a college campus, to Mexico City, where protesters papered the walls of the attorney general’s headquarters with photographs of the missing students and later lobbed rocks through its windows.

Mexican President Enrique Pena Nieto says solving the case is at the top of his agenda. But authorities appear to have made little headway in determining who is behind the students’ disappearance.

According to prosecutors, the students, who attended a rural teachers college known for its radical leftist curriculum, were abducted after a demonstration in Iguala.

The demonstration had been violent, with police opening fire on protesters, killing six. Prosecutors allege that dozens of surviving students were detained by police, who turned them over to members of the powerful Guerreros Unidos cartel. Authorities say a high-ranking cartel member then ordered them killed.

It was initially believed the bodies of the students had been dumped in a mass grave that was discovered shortly after the incident. But this week Murillo Karam said the 28 bodies discovered in that grave were not those of any of the students. Investigators are testing remains found in other mass graves in the area.

Meanwhile, authorities are also searching for the mayor of Iguala, who fled town with his wife shortly after the incident. The mayor, Jose Luis Abarca, is believed to have ties to the gang.

On Friday Guerrero’s state legislature voted to impeach Abarca.