Some of the bodies found in clandestine graves in southern Mexico do not correspond to any of the 43 students who went missing after an attack by gang-linked police, according to an official.
Angel Aguirre, governor of Guerrero state, declined on Saturday to confirm how many bodies had already been identified.
"I can say that some of the bodies, according to the work of forensics experts, do not correspond to the youths from Ayotzinapa", the town where the youths were studying before their disappearance in Iguala, 200km from Mexico City.
Aguirre made the remarks in Iguala, where municipal police have been accused of working with a drug gang in the disappearance of the students on September 26.
The students vanished after police officers linked to the Guerreros Unidos gang attacked buses they had seized in Iguala.
Vidulfo Rosales, a lawyer representing families of the missing students, said he had no information about identification of any of the remains.
He said it was regrettable that authorities had not first informed the families before releasing the information.
Aguirre made no comment about what authorities might have found in other mass graves that were discovered in the same area as the first site on the outskirts of Iguala.
An official in the prosecutor's office declined to confirm the information, stressing that the investigation remains ongoing.
On October 4, a mass grave was found with 28 bodies. The remains had been severely burned, and experts are conducting DNA tests in an effort to identify the dead.
And on Thursday, the discovery of four more unmarked pits with an undetermined number of bodies were disclosed by Mexico's attorney general, Jesus Murillo Karam.
Karam also announced the arrest of four people, raising the total in custody to 34, including 26 Iguala police officers.
Aguirre said on Saturday that no more arrests had been made in the case.
Two Guerreros Unidos hitmen confessed to executing 17 of the students - who are from a teacher training college known as a bastion of protests - and dumping them in the mass grave.
Authorities say the officers shot at buses the students had seized to return home on September 26, prompting a night of violence that left six people dead, 25 wounded and 43 missing.
Surveillance cameras showed several students being taken away in patrol cars.
Iguala's mayor, Jose Luis Abarca, his wife and the public security director are wanted for questioning. They have apparently gone into hiding.
Abarca's wife, Maria de los Angeles Pineda Villa, is the sister of two late members of the Beltran Leyva drug cartel, which founded the Guerreros Unidos.
Fears of a massacre has tarnished President Enrique Pena Nieto's pledge to reduce violence in Mexico and ensure that human rights are respected.
Mexico's drug war has left 80,000 people dead since 2006.