An investigation into the case of 43 missing college students in the southern Mexican state of Guerrero has led to the discovery of at least 60 mass graves and 129 bodies, the Associated Press (AP) reported, citing Mexico’s attorney general’s office.

Although 16 of the dead bodies have been officially identified so far, authorities have not yet connected any of the remains to the students who disappeared in Iguala city last September. Authorities began investigating unmarked graves as a part of the search for the missing students, after the graves were found between last October and May. The actual number of mass graves could be higher.

The students from the Ayotzinapa Teacher Training College in Guerrero -- a state with one of the highest homicide rates in Mexico -- went missing after they were protesting discriminatory hiring practices in Iguala on Sept. 26, 2014.

According to the government, local police handed the students to the members of a drug gang after mistaking them for a rival gang. In November, gang members confessed to killing the students and incinerating their bodies. At least 80 people have been detained so far in the case. The badly burnt body of one student was found at a garbage dump in December, however, families of the rest of the missing students reportedly refuse to believe the students are dead.

Investigators later stated that Jose Luis Abarca, the mayor of Iguala at the time of the incident, turned the students over to the local gang after he feared that they would disrupt his wife’s speech at a conference. Abarca, his wife and dozens of police officers along with some gang members were arrested in connection with the incident.