MEXICO CITY (AP) — Investigators in Mexico announced Thursday they have found 12 bodies in clandestine burial sites in the northern border state of Chihuahua.

State prosecutors said the skeletal remains were found in the township of Ascencion, about 110 miles (180 kms) west of Ciudad Juarez, which is itself across the border from El Paso, Texas.

The investigators started exploring the desolate site on Dec. 18, and in subsequent days, they expanded the search area.

They eventually found 11 separate shallow pits into which a dozen bodies had been dumped. The remains were taken to state forensic laboratories for possible identification and to determine where possible the cause of death.

Drug cartels and kidnapping gangs in Mexico often use such clandestine body dumping grounds to get rid of the corpses of their victims or rivals. The grisly practice has contributed to the enormous problem of missing people in Mexico, who now number about 120,000.

The relatives of most of those missing people are largely left to look for their loved ones by themselves, and they frequently form volunteer search groups that go out into the desert looking for clandestine graves. It was not known if any of those volunteer groups had helped authorities in locating the graves in Ascencion.