Mexican officials have discovered hundreds of skeletal remains scattered on ranches in a stretch of towns along the U.S.-Mexico border as they carried out a wide search to locate missing people.

The remains were burned and extremely hard to identify, said Coahuila state prosecutors' spokesman Jesus Carranza on Monday.

Number of remains, victims unclear

News of the grisly finds came at the same time 12 bodies were unearthed from clandestine graves in the southern Mexico state of Guerrero and about two months after 67 bodies were found in western Mexico. Such discoveries remain common despite government claims that the number of killings has gone down in the past year.

We are still not sure how many skeletal remains and how many victims we are talking about. - Coahuila state prosecutors' spokesman Jesus Carranza

Police in Coahuila haven't said whether an organized crime group is suspected in the skeletal remains, but the area is known to be dominated by the violent Zetas drug cartel. Officers have arrested 10 men, including four police officers suspected of aiding a criminal group, the state attorney general's office said in a press release.

The police operation took place on ranches in 11 different towns around the border city of Piedras Negras, right across Eagle Pass, Texas, after interviews with family members and at least 32 former local officials. Usually, remains like these have been found in mass graves, but these were left on the surface in the region known as Five Springs, Carranza said.

At some of the ranches, investigators found bullet casings as well as barrels of diesel fuel that was likely used to burn bodies.

"We are still not sure how many skeletal remains and how many victims we are talking about," Carranza said. "But this operation was launched to try to locate missing people."

Crime scene contamination worries

An organization that supports families of missing victims has gathered 321 cases between 2007-13 in just Coahuila. Families were worried Monday that the government contaminated the crime scenes because of images from local media showing the use of heavy machinery at the search sites.

"It makes you want to cry. It's unbelievable how they handle evidence. It just complicates the identification of the remains," said Guadalupe Fernandez, a member of Forces United for our Disappeared in Coahuila and the mother of Jose Antonio Robledo, an engineer who went missing in 2009.

The state prosecutors' office said it followed protocols of preservation in crime scene investigations.

In Guerrero state, the attorney general's office said a dozen bodies were found Sunday in the town of Mexcaltepec by military personnel after they received an anonymous tip Saturday night.

In the same state and only a few days before, members of armed self-defence groups found a clandestine grave in the town of Cajelitos near the state capital of Chilpancingo and reported the skeletons of three men and two women.

Two months ago, authorities excavated for several days in recovering 67 bodies that had been bound or gagged in a remote town by Lake Chapala, a popular spot with tourists and American retirees.

It is a region where the Knights Templar and New Generation drug cartels are fighting each other. Local police officers who confessed to handing over people to the New Generation organization led investigators to the scene, officials said.