MEXICO CITY (AP) — Police said Thursday they found 14 sets of skeletal human remains in seven clandestine burial pits in area known as Mexico’s “Thousand Islands.”

Police, soldiers and National Guardsmen used boats to reach four islands in a reservoir in the southern state of Oaxaca, near the border with the Gulf coast state of Veracruz.

State prosecutors said eight of the skeletons were men and six belonged to women. Most of the skulls showed bullet wounds, suggesting the victims had been shot in the head.

Dozens of military personnel provided an escort for investigators in the dangerous area of the Miguel Alemán reservoir, where drug gangs operate.

Armed men detained in an earlier raid gave police leads about the location of clandestine grave sites.

Attempts at developing a tourist trade in the reservoir, which is dotted with hundreds of tiny islets, have foundered because of the widespread violence and kidnapping in the area.