Four new mass graves with human remains have been found outside Mexico City, fuelling fears that the corpses could be some of the 43 missing students who went missing from Iguala, Guerrero state, on 26 September.
It is feared the students have been executed by drug cartels and corrupt police, after a mass grave with at least 28 corpses was found on the outskirts of the town in October.
Officials were led to the new mass graves by four new suspects of the alleged mass killing.
"They say there are remains of students," attorney General Jesus Murillo Karam told AFP, adding that some of the bodies appeared burned.
Authorities have said it will take at least two weeks to identify the bodies through DNA analysis.
Meanwhile, two hitmen confessed to executing 17 of the students and dumping them in the mass grave found last week.
The students, from Isidro Burgos College, went missing after they clashed with police, who shot at a bus that they claim had been hijacked by the youths. The clashes resulted in the death of two students and one person in a taxi.
Mexicans held several protests across the country to urge the government to take action over the unexplained disappereance.
Murillo said 22 local police have been arrested in connection with the incident and the total number of people held is now of 34.
He added that a search for the fugitive mayor of Iguala, Jose Luis Abarca, his wife, and the local head of security was now formally under way.
Mexican President Enrique Pena Nieto vowed on Monday [6 October] to identify those behind the massacre and make sure they are held accountable.
Officials have discovered several mass graves around Mexico in recent years, some of which contained the bodies of migrants.
Hundreds of Central American migrants travel to Mexico each year and they are often killed by drug cartels which control the migrant-smuggling routes.
Last June, Mexican authorities found another mass grave containing at least 28 corpses near a ranch in the town of Tres Valles in Veracruz.