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 September 26, was not yet clear.

Al Jazeera's Rachel Levin, reporting from Mexico City, said that federal authorities were investigating the new leads on Thursday, as President Enrique Pena Nieto tried to deflect criticism of government inaction in finding the 43 students.

On Wednesday, tens of thousands of people held protests across Mexico to demand the return of the missing students, amid fears a police-backed gang executed them.

The young students disappeared after municipal police officers working with a gang shot at buses seized by the aspiring teachers in the Guerrero city of Iguala, and took several of them away in patrol cars.

A mass grave containing 28 unidentified bodies was discovered on the outskirts of Iguala last weekend, in the same location where two hit-men from the Guerreros Unidos gang confessed to executing 17 students.

Authorities said it would take at least two weeks to confirm the identities of the bodies.

The government of President Pena Nieto has faced US and UN calls to solve the disappearance and investigate why gang-linked police attacked the students.

Pena Nieto deployed hundreds of federal forces to take over security in Iguala on Monday and disarm the local police. About 30 investigators from the prosecutor's office have also traveled to Iguala.

The mayor of Iguala, Jose Luis Abarca and his wife, Maria de Los Angeles Pineda Villa, have been suspected of having links to the disappearnce of the students. Both have gone missing.

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.

Guerrero, which is also home to the resort of Acapulco, has been one of the most lawless states in Mexico for years.

Source: Al Jazeera and agencies