Mexico's attorney general confirmed early Wednesday that none of the missing students' remains had been found in a mass grave where investigators had thought they might be buried.

The probe had recovered 28 bodies, but none of the DNA matched those of the 43 college students' who disappeared late last month near the Guerrero state town of Iguala.

Guerrero Governor Angel Aguirre also confirmed the results, adding that "some [of the graves] are from months ago," which could mean they belonged to earlier victims. He also noted that the remains were difficult to identify because they had been burned.

Despite the Mexican government's decision to dismiss the local police force and send in federal agents to enforce law and order, local teachers and students have staged protests in a bid to find the missing 43.

On Monday, hundreds of demonstrators, who were demanding Governor Aguirre's resignation, ransacked and burned a local government building in Guerrero's capital city, Chilpancingo.

The case of the missing the students began on September 26, when police shot at buses which had been allegedly hijacked by students. The shootout left six people dead and dozens wounded. The 43 students could not be accounted for following the violence.

Details which have emerged since the incident have pointed to collaboration between local police and drug gangs in the disappearance.

Authorities have discovered 10 mass graves outside of Iguala in the weeks that followed. They have also detained nearly 30 police officers from Iguala and 14 more from the neighboring town of Cocula. At least four members of the gang Guerreros Unidos have also been detained.

kms/rc (AP, AFP, Reuters)