SANTA ELENA, Guatemala—El Petén province, a vast stretch of wilderness in northern Guatemala known for its rainforests and stunning Mayan pyramids at Tikal, is fast becoming a stronghold for a notoriously bloodthirsty Mexican cartel.

Last month, soldiers entered a cattle ranch in El Petén to find the remains of a brutal human slaughter: Twenty-seven bodies strewn across the property and a pile of heads thrown over a fence. On a wall was a message written in blood and signed "Z200," a moniker authorities say belongs to a local...