Mexican authorities on Monday said they had arrested four drug gang members involved in the kidnapping of dozens of student teachers who disappeared last month and are feared massacred.
The announcement came as local media reported that a mass grave has been discovered in a trash dump outside mountain town of Cocula, near Iguala in the southwestern state of Guerrero, where 43 students disappeared after they clashed with police and masked men on Sept 26.
Mexico's Attorney General Jesus Murillo said the four members of the Guerreros Unidos gang had been involved in the kidnapping of the students, which has sparked nationwide protests and undermined President Enrique Pena Nieto's claims that Mexico is becoming safer under his watch.
"Today we now have those who organized the disappearance of these youths," Murillo said.
He did not elaborate on the media reports of another mass grave site, but he said that the suspects had identified a crime scene where reporters would be taken on Tuesday.
The attorney general has said Iguala's mayor and his wife were the probable masterminds of the disappearance and ordered local police forces to stop the students from disrupting a political event to launch a campaign for his wife to succeed him as mayor.
Forensic anthropologists are still checking the remains of dozens of corpses found buried on a hillside outside Iguala, but so far none of the students' have been found.
Federal authorities have arrested more than 50 people in connection with the incident, including dozens of police who have links to the Guerreros Unidos gang, which translates as United Warriors.
The disappearance of the students has triggered massive protests from Mexico City to the Pacific seaside resort of Acapulco, overshadowing Pena Nieto's bid to restore order in Mexico and shift the focus away from endemic gang violence and onto economic growth in Latin America's No. 2 economy.
Around 100,000 people have been killed in gang-related violence since the start of 2007.
