Police in Mexico say they have found at least 19 bodies in an abandoned mine shaft in Hidalgo, near Mexico City.
The security forces said they found the decomposing remains after a tip-off from two alleged members of a drug cartel they had arrested on Friday.
The two men also named nine police officers who they say worked for their cartel, Los Zetas.
The nine have been arrested and are being questioned, while police continue to search the area for more bodies.
They say the two alleged cartel members told them there were more human remains at at least four other locations in the area.
Since President Felipe Calderon came to power four years ago, more than 28,000 people have died in drug-related violence in Mexico.
And while turf wars between rivalling drug cartels have been particularly fierce in the north of the country, mass graves are also increasingly being discovered in other states.
In May, police found 55 bodies in an abandoned mine near Taxco, in Guerrerro state.
And on Sunday, four decapitated bodies were found hanging from a bridge in the affluent central city of Cuernavaca.