MEXICO CITY (AP) — Volunteer searchers in northern Mexico led authorities to a series of grisly finds: 17 bodies or skeletal remains buried in the backyards and patios of houses in a low-income housing development, prosecutors in the northern border state of Sonora said late Sunday.

They said the bodies had been stacked in four clandestine burial pits, and said searches would continue Monday at other homes.

The prosecutors office said the finds were made over the weekend at abandoned houses in the city of Ciudad Obregon. It said the victims had apparently been killed between six months and a year ago.

The state prosecutors' office said the bodies would be subject to genetic and specialized forensics tests to identify them.

Photos posted by the prosecutors' office showed searchers digging by hand and with a backhoe in the yards, and in some cases under the foundations of the homes.

While it is more common for bodies to be buried in pits in vacant lots or outside of towns, some drug gangs in Mexico use abandoned or rented dwellings as execution chambers for kidnap victims or suspected rivals, and simply bury the bodies under the floors or in the yards before abandoning the dwellings.

Sonora has been locked in a bloody three-way turf battle between rival drug gangs, and the discovery of clandestine burial pits has become increasingly common. The turf battles involve rival factions of the Sinaloa cartel and gangs allied with fugitive drug lord Rafael Caro Quintero.

Last week, volunteer searchers found 11 bodies in clandestine burial pits just a few miles from the U.S. border.

The government of Sonora state said the bodies included nine men and two women were found in a series of pits, many holding several bodies, in San Luis Rio Colorado, across the border from Yuma, Arizona.

Volunteer search teams made up of relatives of disappeared people led authorities to the pits in a stretch of desert near a garbage dump.

Relatives of the disappeared have to conduct their own searches in many parts of Mexico, because police are unable or unwilling to do so.

Mexico has more than 98,356 disappeared, according to government data. Most are thought to have been killed by drug cartels, their bodies dumped into shallow graves, burned or dissolved.

The government has struggled to identify even the bodies that have been found. Some 52,000 await identification.