Mexican authorities found 16 bodies in three clandestine graves on the outskirts of Monterrey, an industrial city that has seen an upsurge in drug cartel-related violence, officials said Wednesday.

David Perales, spokesman for Nuevo Leon state's investigative agency, said the burial sites were located on an abandoned ranch in the township of Juarez, the same area where 51 bodies were found in mass graves in 2010.

The is no immediate information on the victims' identities or those of their killers, but authorities said previously the 2010 burials were probably carried out by a drug gang.

Many farmers and ranchers in the region have abandoned rural properties in the face of drug gang violence; cartels frequently take over the ranches to use them as operating bases, training grounds or clandestine burial sites.

Authorities said they went to the ranch outside Monterrey Tuesday after drug gang suspects revealed the burial sites during questioning.

They found 15 human skulls and other bones. The bodies appeared to have been buried there at least 8 months ago.

Further north in the border city of Piedras Negras, many parents kept their children home from school Wednesday following a fierce hour-long battle between gunmen and police. One female officer was killed and six people were wounded.

Tuesday night's shootout left an atmosphere of fear across the city of Piedras Negras, across the border from Eagle Pass, Texas.

Authorities have not identified the gunmen involved in the attack.

Prosecutors in Coahuila state said one of the wounded police officers is in serious condition.

The attackers fled following the shootout, leaving behind a large quantity of weapons, including assault rifles, rocket-propelled grenades and grenade launchers.

(This version CORRECTS spelling of name to Perales)