He said Felipe Rodríguez Salgado, who was arrested Jan. 15 and identified as a leader of Guerreros Unidos, told investigators that he had been ordered to get rid of the students because they had been infiltrated by a rival gang, Los Rojos. Some of the students were singled out as infiltrators, but the attorney general left unclear why all of the students had been killed and not just the suspected rivals.

Mr. Murillo Karam added that no evidence had emerged to suggest that any of the students had been involved with Los Rojos.

The students were rounded up and brought to a site near the dump. Some were interrogated but kept telling their abductors they were students before they were killed, Mr. Murillo Karam said.

Despite the recovery of thousands of pieces of remains, a sophisticated laboratory at the University of Innsbruck in Austria that Mexico enlisted in the investigation could only positively identify one student, Alejandro Mora. The rest of the remains taken to the lab were too damaged to be identified and would need further testing, he said.

Mr. Murillo Karam also addressed suggestions that the fire could not have been intense enough to destroy the bodies, saying it had been made from tires, diesel and wood and had burned for days in a ravine near the town dump of Cocula, Mexico. The ashes were then bagged and thrown into a river.

Human rights groups and parents of the missing students have accused the government of being slow to investigate and of mishandling the case. The parents have vowed to continue to look for their children, and have said they mistrust the authorities and do not believe their account.

Melitón Ortega, the father of Mauricio Ortega, 17, one of the missing students, said that the parents had not been informed of the new evidence that the government presented on Tuesday.