Mexican authorities have found four more clandestine graves containing charred human remains at a site in the restive southwest of the country, where officials fear missing students were massacred by gang members and police.
Attorney General Jesus Murillo said on Thursday the motive behind the apparent massacre of dozens of student teachers, who went missing after clashing with police in Iguala in the volatile, gang-ridden state of Guerrero on Sept. 26, was not yet clear.
The new find brings the number of graves discovered on the outskirts of Iguala to 10. Murillo said DNA samples were being taken from their relatives to check against the remains found.
Thousands marched through the Mexican capital on Wednesday to demand the government find out what happened to 43 missing students.
Murillo said a search for the fugitive mayor of Iguala, Jose Luis Abarca, as well as his wife and the local head of security was now formally under way.
President Enrique Pena Nieto vowed on Monday to identify those behind the massacre and make sure they face justice.
Guerrero's attorney general, Inaky Blanco, said on Sunday that 28 bodies had been found at a mass grave site so far, adding it was "probable" that some of the missing 43 students are among the remains.
Their disappearance is only the latest incident of bloody violence that is becoming an increasing problem for Pena Nieto, who has sought to shift attention away from Mexico's gang wars to the economic reforms he has pushed through Congress.
Some 22 local police have been arrested in connection with the incident. Murillo said four more people had been detained in connection with the case, taking the total number of people held to 34.
Pena Nieto took office two years ago, pledging to end a wave of violence that has killed about 100,000 people since the start of 2007. Though homicides have fallen on his watch, other crimes have increased, including extortion and kidnapping.