Mexican marines patrolling a lake along the border with Texas discovered a drug gang camp on an island, provoking a gunbattle that left 13 people dead, the navy said Monday. Investigators in a different northern state reported finding 11 decapitated bodies and exhumed 22 remains from mass grave sites where they have found 179 dead people since last month. One marine and 12 suspected gunmen of the Zetas drug cartel were killed in the battle Sunday on Falcon Lake in the northeastern state of Tamaulipas, the navy said in a statement.