State Prosecutor Jorge Winckler says the clandestine burial pits appear to contain the victims of drug cartels killed years ago.
In an interview Tuesday with the Televisa network, Winckler did not specify when the skulls were found or by whom.
But victims' advocacy groups like Colectivo Solecito have excavated and pressed authorities to excavate such sites to find missing loved ones.
Winckler said excavations have covered only a third of the lot where the skulls were found, and more people may be buried there.