Suspected gang members have confessed to killing more than 40 students missing for six weeks, Mexican Attorney General Jesus Murillo has said.
He said three alleged gang members claimed the students were handed over to them by police.
They said some were already asphyxiated and they shot the others dead, before setting fire to all the bodies, the BBC reported.
A total of 43 students went missing after clashing with police on 26 September in the town of Iguala.
The suspects from the Guerreros Unidos drug gang were recently arrested in connection with the disappearances.
Relatives of the missing said they had been told that six bags of unidentified human remains had been found along a river near where the students vanished.
Murillo warned that it would be difficult to identify the charred remains and that authorities would continue to consider the students as missing until DNA tests confirmed the identities.
Previous searches have uncovered mass graves in the area, but initial tests suggested they did not contain the remains of the students.
About 15 of the students were already dead when they arrived and the rest were shot, according to the suspects.
Murillo said the bodies were then burned with petrol, tires, firewood and plastic in an inferno that lasted for 14 hours.
“The fire lasted from midnight to 2pm the next day. The criminals could not handle the bodies (for three hours) due to the heat,” he said.
He said that the suspects then crushed the remains, stuffed them into bags and tossed them in a river.
The case has shocked Mexico. Thousands have staged protests over what they say is collusion between officials and organized crime, along with government inaction.
President Enrique Pena Nieto has faced widespread criticism and on Friday he vowed to hunt down all those responsible for the “horrible crime”.
The students from a teacher training college in Ayotzinapa, in Guerrero state, had travelled to nearby Iguala to protest against what they said were discriminatory hiring practices, and to collect funds for their college.
But they went missing after clashes with the police.
Six people were also killed after police opened fire and witnesses described seeing the students being bundled into police cars.
More than 70 people have been arrested in connection with the disappearances, including the Mayor of Iguala, Jose Luis Abarca, and his wife, Maria de los Angeles Pineda, who were detained in Mexico City on Tuesday.
Mexican officials accused Abarca of ordering police to confront the students to prevent them from disrupting a public speech given by his wife.
