Authorities have found four more graves containing burned human remains at a site where officials believe dozens of missing students were murdered by gang members and police, Reuters reports.

Forty-three students went missing in the violence-plagued state of Guerrero on Sept. 26. Since then authorities have found a total of 28 bodies in 10 graves on the outskirts of Iguala, a town within the same region. Iguala Mayor Jose Luis Abarca, his wife and the local head of security have gone missing. Attorney General Jesus Murillo said Thursday that they do not yet know the motive for the suspected massacre of the teachers and students who went missing and that the search for Abarca and the other fugitives is underway. Meanwhile, police are testing the bodies using DNA from family members of the missing.

Thousands of protestors marched through Mexico City on Wednesday, demanding answers. President Enrique Pena Nieto vowed on Monday that whoever murdered the students will be brought to justice. Twenty-two police have already been arrested in connection with the incident, and four more people have been detained in connection with the case.

[Reuters]

Write to Eliana Dockterman at eliana.dockterman@time.com.